gitk: Add accelerators to frequently used menu commands

gitk: Add accelerators to frequently used menu commands

This commit documents keyboard accelerators used for menu
commands in the menu, as it is usually done, and adds some
more, e.g. F4 to invoke Edit View (or New View if the current
view is the un-editable "All files" view).

The changes include a workaround for handling Shift-F4 on
systems where XKB binds special XF86_Switch_VT_* symbols
to Ctrl-Alt-F* combinations.  Tk often receives these codes
when Shift-F* is pressed, so it is necessary to bind the
relevant actions to them as well.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>