Include a desktop file

The Debian package ships a vite.desktop file, which creates the the menu entry for the application. However, Debian policy recommends to forward this file upstream (other distro may also use it).

Including the file in our source tree is not difficult, what is more complicated (maybe just because I don't know how to do it) is to configure CMake to generate it (replace the path of the icon and the binary) and install it at the right place.

In the same time, it would be nice to check if the desktop file requires a StartupWMClass entry, like Dia does.