Use the legacy GDK keyboard definitions in gps.c

display-gtk.h has a check if gdkkeysyms-compat.h or gdkkeysyms.h
has to be used. In gps.c, from such as "GDK_Up" has to be used,
so that this file compiles on older GTK.

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Added the OS dependent function subsurface_launch_for_uri()

Opening URI addresses from Subsurface does not work on Windows using
the latest GTK bundle from the Gnome website. The reason lies in GIO
and GLib and how it obtains assigned applications for protocols and MIME
types.

While gtk_show_uri() should be viable for both linux.c and macos.c,
in windows.c ShellExecute() is used, which provides proper support
for the URI calls.

subsurface_launch_for_uri() returns TRUE on success.

Signed-off-by: Lubomir I. Ivanov <neolit123@gmail.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Attempt to override the URI handler in the about dialog

The previous patch in this series adds the function
subsurface_launch_for_uri(), which has to be called from the about
dialog instead of the default function for the URI calls to work
on Windows - i.e. opening links from the dialog.

gtk_about_dialog_set_url_hook() is a deprecated method, but seems
to be the only reasonable way to make the override between GTK 2.20 -
2.24 possible. The "activate-link" signal is recomended instead,
but is not portable for GTK bellow 2.24.

An alternative would be to somehow hack into the GtkAboutDialog
and override the callback for each clickable link.

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