gui: Use comprehensions when initializing lists, dicts, and sets

[echoix]

Analogous to #4909, but for files in gui/

All combined changes come from this branch (where it gets rebased): https://github.com/echoix/grass/tree/pyrefact-gui (echoix#335)

[echoix]

For the remaining, i'll file them in later, it is getting late, and I'm not sure that I can create a PR that is completely free of conflicts for the PR grouping the context managers (analogous to #4908). There's also one for early exits (can be split in two: functions and loops, similar to #4910), and one more for the remaining of changes in gui (analogous to #4911).

[echoix]

Tomorrow I also want to see how many wx.Dialogs that I can use a context manager to handle the dlg.Destroy cleanly in all cases (like the second example here: https://docs.wxpython.org/wx.Dialog.html#phoenix-title-modal-and-modeless).
Ideally some of the pending GUI PRs should be merged to not work for nothing

[ninsbl]

Looks good. It is quite a sizeable change though, so I hope I did not overlook anything. I assume you leave the %-string formating for another round.

[echoix]

Looks good. It is quite a sizeable change though, so I hope I did not overlook anything. I assume you leave the %-string formating for another round.

Yes, there's like 2400 UP031 and 382 UP032, so it's not really a good thing to shove into the rest.

The PR for early exit is a bit harder to review, so I made sure to keep (manually replace) the empty lines to have a "landmark"