Adds checkboxes to the document tree which makes it possible to
select multiple document objects on devices without physical keyboard.
These checkboxes are opt-in and can be enabled/disabled from:
Edit -> Preferences -> Display -> Navigation -> Selection
Forum thread for feature the request:
https://forum.freecadweb.org/viewtopic.php?f=34&t=53065
We need to iterate the tree to add/remove the selection boxes when
the enable-setting has been changed.
The size for the first column in the tree is adjusted to fit both
name and optional checkbox.
