Allow undoing with Ctrl+z in text areas/boxes

[forteller]

When a text area/box is selected, the expected thing is that keyboard shortcuts affect the text. But if you hit Ctrl+Z, the map itself hijacks the shortcut, and you're asked if you want to undo all edits you've done in the map, instead of the text being undone.

This can be disastrous, especially until #1107 is solved.

Thank you for your great work!