Refactor and fix icon locations

Created by: jonathonf

This refactors out the duplicated desktop launchers, replacing them with a template which is used by the Makefile to generate a launcher for each application.

It also fixes the default icon installation location to remove the implicit dependency on hicolor-icon-theme and place them in the default icon search path.

Packagers could use e.g. imagemagick to rescale the 1024x1024 icon and install smaller icons into the appropriate hicolor icon locations.