[main-] handle position args as +sheet:subsheet:col:row

[midichef]

Two bug fixes:
3ea655b - fixes setting column position in sheet
Currently the starting column cannot be set, only the row.
repro command: vd sample_data/sample.tsv sample_data/StatusPR.csv +2:2
This occurs because the column position changes too early, before the sheet is loaded.

1aa2d98 - fixes setting column position in subsheet
A similar problem, changing position before the subsheet is loaded.
repro: vd ~vdsrc/sample_data/employees.sqlite +:emp:5:2

And then some functionality changes:
d16743d - change arg parsing, from row:col to col:row
Strings of the form a:b are parsed as row:column. This commit changes it to column:row.
Previously, I discussed some pros and cons of this change.
This is technically a breaking change, but as I noted in that comment, the form +a:b has not worked since 2.9 so it's not likely that anyone is currently using that syntax. However, if they are using +:a:b with the extra colon, this will indeed be a breaking change for them.
repro: vd ~vdsrc/sample_data/sample.tsv +:1:4
(note the : before the 1. It's needed (on develop) to demonstrate the column-positioning bug, until the 2 commits above are applied.)

0a8f6ee - handling args of form +a:b vs +:a:b.
Comments in the code suggest that '+col:row' should apply to only the last sheet on the list, vs. adding a colon, '+:col:row', which should apply to all sheets:

visidata/visidata/main.py

Line 90 in c01c2b5

'Return (startsheets:list, startrow:str, startcol:str) from *arg* like "+sheet:subsheet:col:row". Empty sheetstr in startsheets means the starting pos applies to all sheets.'

(The suggestion comes from the phrase Empty sheetstr in startsheets. An empty sheetstr happens only when arg.split(':') has more than 2 items and pos[:-2] is '', which happens when arg follows the pattern +:a:b:

visidata/visidata/main.py

Line 105 in c01c2b5

startsheets = pos[:-2]

Currently the behavior is the reverse: +col:row applies to all sheets.
repro: vd sample_data/sample.tsv sample_data/StatusPR.csv +:2:2
and look at the cursor position on each sheet.
This commit changes +:a:b to apply to all sheets, and +a:b to apply only to the last sheet.

da52ac3 - improve error for invalid sheets
Attempting to index a sheet that does not exist, as with:
repro: vd sample_data/sample.tsv +10:0:0
gives a traceback with IndexError: list index out of range.
This commit handles the error and shows a status message of no sheet "10".

a0d4913 - handling subsheet names that are integers
For consistency with sheets/rows/cols, this commit allows indexing subsheets with integers, like:
numeric-subsheet-names.vds.txt
repro: vd -f vds numeric-subsheet-names.vds.txt +0:3:0:0
This is also a change that breaks current behavior, for a subsheet that happens to have a name that is an integer. Such a subsheet would become unable to opened by using its name, as with sheets 5, 6, 7, and 8 in numeric-subsheet-names.vds.txt; +0:5:0:0 currently works, but after this commit only +0:0:0:0 would work.

Thoughts? I'm not so worried about breaking changes, because I don't think the vd + feature is heavily used.

Just to write out all the cases, here's what the args would mean after all these commits:

    +2         = row 2
    +1:        = last sheet; col 1
    +:1:       = all sheets; col 1
    +:2        = last sheet; row 2
    +1:2       = last sheet; col 1, row 2
    +:1:2      = all sheets; col 1, row 2
    +0:1:2     = sheet 0; col 1, row 2
    +:a:1:2    = last sheet, subsheet a; col 1, row 2
    +0:a:1:2   = sheet 0, subsheet a; col 1, row 2
    +0:a:b:1:2 = sheet 0, subsheet a from sheet 0, subsheet b from sheet a; col 1, row 2
    +0:0:1:2   = sheet 0, first subsheet of sheet 0, col 1, row 2
    +1::       = 2nd sheet, col 0, row 0
    +-2::      = 2nd sheet from last, col 0, row 0

[midichef]

I fixed the type hint that was breaking in Python 3.8, by adding ' quotes to make it a string:
https://github.com/midichef/visidata/blob/bcf812f0052d54f25a6c1c5d7734f9cec2e86a06/visidata/main.py#L89
I don't understand type hints enough to know what the string does, I'm just doing it because it worked in sort.py.

visidata/visidata/sort.py

Line 50 in c01c2b5

def sortkey(sheet, r, ordering:'list[tuple[Column, bool]]'=[]):