[PATCH 0/3] added URI support on Windows

[Lubomir I. Ivanov]

[PATCH 1/2] Added the OS dependent function subsurface_launch_for_uri()

The patch commit explains briefly that URI support for Windows is a bit
broken primarily due to GIO.

subsurface_launch_for_uri() can now be used to properly handle
passed URI's in a multiplatform way.

Since #ifdefs are not used directly in gtk-gui.c, the linux.c and macos.c
implementation is the same.

Examples of future use that may remain broken on Windows without this
patch:
- opening URLs other than the ones in the about dialog for some sort
of web integration
- opening dive XML files to be edited in a "default text editor"
- opening local images / media

[PATCH 2/2] Attempt to override the URI handler in the about dialog

This patch provides an example of the above concept fixing the homepage link
in the about dialog, which is pretty much the only case where this is needed
at this point.