Tree: rounded selection

[DevCharly]

This PR enables using "rounded" selection for tree selection.

This is not yet used in any theme, but intended to be used for macOS themes (see PR #533) and future Windows 11 style themes.

Perhaps this should also be used for existing themes to make tree selection more beautiful.

Example

UI properties for above screenshot (see Application properties files):

Tree.selectionInsets = 0,1,0,1
Tree.selectionArc = 6

[DevCharly]

Non-wide selection is also supported:

Tree.wideSelection = false
Tree.selectionInsets = 0,0,0,0
Tree.selectionArc = 6

Or with a gap:

Tree.wideSelection = false
Tree.selectionInsets = 1,0,0,0
Tree.selectionArc = 6

[DevCharly]

The PR is merged. Don't know why GitHub did not recognize it.