This fixes#51. The desktop file is now exactly the same as the window class,
which is also now corrected to org.polymc.polymc. The file capitalization
is also consistent with other Freedesktop files as well.
With the Property 'PrefersNonDefaultGPU', you can signal to dekstop environments like GNOME that you want to run the application using the more powerful secondary graphics card, if such a card is available. Specification: https://specifications.freedesktop.org/desktop-entry-spec/latest/ar01s06.html
With this change, you won't have to include a special prime-run script.
