[kcms/keyboard] Some Advanced basics

Clip the TreeView, not the ScrollView

ScrollView is an arbitrary item which may or may not have graphical parts outside its view bounds, but its flickable content item is the one that should be clipped.

Use appropriate state change handler for CheckBox

Binding assignments to the raw value change signal has a potential of causing binding loops. CheckBox users should listen to toggled() signal instead.

Use a proper columnWidthProvider instead of faking implicit size

Binding child component's implicit size to its parent's explicit size is completely backwards to how sizing is supposed to work.

As a bonus, we don't have to bind to width changes to call forceLayout. because TableView will recalculate and update the layout anyway.