git-gui: fix size and position of window panes on startup

git-gui: fix size and position of window panes on startup

The themed panedwindow needs to have the sash position set after the
widget has been mapped therefore apply this setting in the Map event
binding. To avoid visible redraws as the application is constructed
the main window should be withdrawn until all the widgets have been added

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