logic analyzer: external clock mode

[eurofun]

The logic analyzer design supports an external clock mode (D0 line). As I found out after some investigation this mode
can already be used with scopy running by executing the following command:
iio_attr.exe -u ip:192.168.2.1 -d iio:device8 clocksource external

Maybe include a checkbox to turn this on without the need for the command. It can be useful for reducing the data rate if only the information at the rising clock edge is needed. The downside is that the D0 line appears as always high, making it difficult to use decoders that rely on a clock trace and that the displayed timing info is of course wrong. So probably an advanced option...

A little off-topic: navigating the captured data is so much easier in recent pulseview versions (scroll to zoom), I often export the data as .vcd file and look at it using pulseview just for that reason. And in my opinion the 'instruments' should stay open for looking at the data even when the device is disconnected. Apart from that a great project!