Store the main window maximized state in the configuration

In save_window_geometry() use gdk_window_get_state() to retrieve
the window maximized state and store it in the user configuration.
Then in restore_window_geometry() restore the maximized state with
gtk_window_maximize(), if needed.

To achieve good results we also need to override "window-state-event"
for the main window and define an extra couple of configuration parameters:
"vpane_position_maximized" and "hpane_position_maximized".
Then store two different pane states for maximized and restored window each
time the user makes a switch.

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
gtk-gui.c: Some small whitespace indentation fixes

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>