A few adjustments to file/dir open dialogs

[Krakean]

a) Added Backspace key support for Tree-based file dialog.
b) Fixed issue inability to select a folder in project manager (always previous folder was selected instead). Video: https://i.imgur.com/iQe8XAN.gifv (#13242, #13203)
c) Open Directory mode: changed "Open" to "Select Current Folder"
d) Block "Open" button when inappropriate content is selected (for example, file when in open folder mode, or folder when in open files mode)
e) List-based file dialog now support item deselecting (when you click on empty space). Video: https://i.imgur.com/i90oId9.gifv

P.S. Require PR #13245 to function/compile - for d) & e)

Fixes #13203.

[akien-mga]

Overall this is good, but I see a couple issues that should be addressed IMO:

• The new "Open" button text "Select Current Folder" does not make sense when you have selected a subfolder and want to pick this one (there are two workflows: either you browse in the final folder and expect it to be the "current" one that will be picked if nothing is selected, or you browse to its parent and select it in the ItemList before pressing Return/Open). So the text should maybe be set dynamically based on the context (item_list->is_anything_selected(), as done already for the files mode).
• It's confusing that .. is selected by default but ignored, and that pressing "Select Current Folder" with .. selected opens the current folder. I agree that opening the parent folder is even worse, but that's still bad UX. My proposal:
  • Remove .. just like . was removed already.
  • Add a "Parent folder" button to replace it
  • When entering a folder, nothing should be autoselected (i.e. it should default to "Select Current Folder" by lack of selection). The cursor key Down should then select the first entry of the ItemList.

If you agree with this proposal and can implement it quickly, that would be best, as this is the main blocker for 3.0 beta IMO.

[akien-mga]

Actually I'll merge this version once the CI is happy, since it fixes an annoying bug. I'd still be interested in a follow-up PR with the enhancements I mentioned above. :)

[Krakean]

@akien-mga Completely agree with proposal, will implement updated(or new one if this one will be merged already, which is even preferably I suppose) in Monday before 14pm Europe time.