gitk: use $config_variables as list for save/restore

gitk: use $config_variables as list for save/restore

gitk includes many user defined configuration variables, has all of
these are listed in $config_variables. But this list is not used to
define the variables to be loaded, saved, or restored when cancelling
the configuration dialog, and developers must maintain separate lists of
variables for these purposes. This leads to unnecessary errors and merge
conflicts. Let's replace those separate lists with $config_variables to
make maintenance easier.

While we are on topic, sort the list of names in $config_variables.
This makes it simpler to scan and has fewer chances of conflicts
when new names are introduced.

Helped-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Mark Levedahl <mlevedahl@gmail.com>