LinGui: fix race condition crasher in subtitle/audio lists

clearing a tree view caused selection to change while clearing (and
before clearing is complete). The selection change triggered an update
that attempted a recursive tree view clear.  Since the first clear
leaves the tree view in an inconsistant state till it finished, this
causes a crash.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6083 b64f7644-9d1e-0410-96f1-a4d463321fa5