Implement kayboard navigation in widget.Tree

[Jacalz]

Description:

This implements the Focusable interface for Tree Nodes. Added keyboard navigation:

• Navigate between tree nodes using tab and shift + tab.
• Select a tree node by pressing space.
• Open a branch by pressing right and close using left.

For #1515

Opening as draft for now until I have implemented tests.

Checklist:

• Tests included.
• Lint and formatter run with no errors.
• Tests all pass.

[coveralls]

Coverage: 61.993% (-0.04%) from 62.031% when pulling 9361045 on Jacalz:tree-focusable into c0e25eb on fyne-io:develop.

[andydotxyz]

It seems like up/down would pair better with left/right than tab/shift+tab do...

[Jacalz]

But the focusing controls that landed in the List widget a while ago just uses Tab?

[andydotxyz]

But the focusing controls that landed in the List widget a while ago just uses Tab?

Ah. Oops. On playing with it I don't think that is the best behaviour, it is just the default because we didn't put key handling smarts into List.
Unfortunately it needs work on List as it will only tab through visible widgets - it really should handle moving up and down the list of items full, not just those rendered.

[Jacalz]

I'll close this and work on some other items that I prioritize higher. My backlog is getting rather big and with my limited spare time, I prefer to work on improvements that I will be making use of in the near future. Sorry.