[Gtk] refactor layout system measuring

[lytico]

issue to fix

the layout system doesn't work properly.

it fails eg. for LayoutView, Label, CollectionView's

possible solution

a suggestion to handle it:

implementation as in Gtk4

for each Widget that has it's own size handling, eg. LayoutView, implement a method like

https://docs.gtk.org/gtk4/vfunc.Widget.measure.html

and implement it using this rules:
https://docs.gtk.org/gtk4/class.Widget.html#height-for-width-geometry-management

then call measure in overrides for
OnGetPreferredHeight/OnGetPreferredWidth/OnGetPreferredWidthForHeight/OnGetPreferredHeightForWidth

decorate all that Widget with an interface IWidgetMeasurable

and call it in https://github.com/lytico/maui/blob/4e98afc5e53bdbc469b4e80edee2ac8fb0718386/src/Core/src/Platform/Gtk/WidgetExtensions.cs GetDesiredSize:

if(platformView is IWidgetMeasurable measurable){
        var size = measurable.Measure 
        ....
} else {
      // existing code
}