gitk: small quality of life improvements for themed Tk

[sommer]

These three closely related patches build on https://lore.kernel.org/git/pull.1835.git.1733767737.gitgitgadget@gmail.com to:

1. also allow changing text wrapping preferences for users that do not use themed Tk (that is, users that have turned the "use themed widgets" preference off);
2. no longer change the background color when using themed Tk (the default) as - at least on the platforms I tried - most widgets and the main window do not change their background color anyhow, whereas some widgets do, leading to a messy looking UI where different widgets seemingly assume different background colors; and
3. use the now-available uicolor preference to draw a "sash" (grab handle) between the many panes of the main window when using themed Tk; on the platforms I tried, themed Tk was both the default and it was not rendering any sash, thus requiring users to guess where to click to resize window panes. Manually setting uicolor to a platform-dependent color (see https://www.tcl-lang.org/man/tcl/TkCmd/colors.htm#M4) in the config file works to get sashes that automatically adapt to light mode and dark mode.

cc: Johannes Sixt j6t@kdbg.org

[sommer]

/preview

[gitgitgadget]

Preview email sent as pull.1909.git.1745855598.gitgitgadget@gmail.com

[sommer]

/submit

[gitgitgadget]

Submitted as pull.1909.git.1745855773.gitgitgadget@gmail.com

To fetch this version into FETCH_HEAD:

git fetch https://github.com/gitgitgadget/git/ pr-1909/sommer/master-v1

To fetch this version to local tag pr-1909/sommer/master-v1:

git fetch --no-tags https://github.com/gitgitgadget/git/ tag pr-1909/sommer/master-v1