Adds support to install Cura to an alternative path, while generating a correct .desktop file for it.
* Renaming cura.desktop to cura.desktop.in
* CMAKE: Generate .desktop from .desktop.in
* Replace hardcoded paths with FULL_PATHs
* Tell CMake to generate cura.desktop
* CMake: Set full paths for .desktop generation
* It tells CMake where to find the .desktop.in is and where to store the result.
