gitk: Use the builtin panedwidget size management functions

gitk: Use the builtin panedwidget size management functions

Since Tk 8.5 the paned widget can be told which panes should expand using
the -stretch pane option. The ttk paned widget has a -weight option. This
patch makes use of these rather than handling the configure event which
currently causes the pane positions to creep during resizing and works
very poorly when switching between zoomed and normal views on windows.

Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>