Don't use tree style to draw focus in DrawItemSelectionRect() in wxGTK.

Don't use tree style to draw focus in DrawItemSelectionRect() in wxGTK.

Using gtk_paint_focus() with a tree widget style did a clearly wrong thing
with Clearlooks theme: instead of drawing a focus rectangle it drew a
background with a shadow overflowing the specified rectangle. This resulted in
junk being left when the selection was changing in wx{List,Tree}Ctrl.

Just use the widgets own style instead as this seems to work just fine. After
this change the code for focus drawing in DrawItemSelectionRect() became
identical to the code of DrawFocusRect() so just call the latter from the
former instead of duplicating its code.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64878 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775