Have "make install" act more as expected for a desktop application

Have "make install" act more as expected for a desktop application

I'm trying to get subsurface to get closer to becoming a "regular desktop
application"; so far this is based on the recommendations and guidelines
on OpenSUSE and Fedora.

The icon is now named subsurface.svg and make install installs it in the
correct location. At runtime subsurface first checks if an icon is
installed and if it is it uses that - otherwise it falls back to the old
code that tries to read the svg file from the current directory.

We also install a subsurface.desktop file

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>