gitk: Implement "permanent" views (stored in ~/.gitk)

gitk: Implement "permanent" views (stored in ~/.gitk)

With this the user can now mark a view as "permanent" and it will
appear in the list every time gitk is started (until it is deleted).
Also tidied up the view definition window, and changed the view
menu to use radiobuttons for the view selections so there is some
feedback as to which is the current view.

Signed-off-by: Paul Mackerras <paulus@samba.org>