Make node selection in ui.tree optional, improve access to node collections

[falkoschindler]

In #1924 (comment) we noticed that ui.tree toggles node selection when clicking on a label, even though a QTree usually toggles expansion in this case. Only if the "selected" prop is set, clicking a node label will toggle its selection instead. But since ui.tree always sets "selected" to an empty list when initialized, the user can't expand nodes by clicking on their labels.

A workaround is to remove the "selected" prop:

del tree._props['selected']

It would be nice to be able to configure the behavior via parameters, e.g. selection: bool = True.

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While at it we could also think about public access to the current list of selected, expanded and ticked nodes. This could be done very similar to ui.table:

nicegui/nicegui/elements/table.py

Lines 139 to 147 in 2874555

	@property
	def selected(self) -> List[Dict]:
	"""List of selected rows."""
	return self._props['selected']
	@selected.setter
	def selected(self, value: List[Dict]) -> None:
	self._props['selected'][:] = value
	self.update()