Add GridLines property to List/Tree views
convert ListBox.GridLines to bool
since ListBox has no columns and no vertical grid lines
Rename GridLines property to GridLinesVisible
Merge remote-tracking branch 'mono/master' into sevoku/add-list-gridlines

Conflicts:
	Xwt/Xwt.Backends/IListBoxBackend.cs
Merge pull request #338 from sevoku/add-list-gridlines

Add GridLines property to List/Tree views