MatSelect improvements (#1872) * Added `MatSelect:Clear` * Added `MatSelect:SelectedItemPaintOver( x, y )` * `MatSelect:AddMaterial*` methods now return the created `DImageButton` * Added `local pnl = MatSelect:FindMaterialByValue( value )` * Added `MatSelect:SelectMaterial( pnl )` --------- Co-authored-by: Rubat <3299036+robotboy655@users.noreply.github.com>