Fix Freedesktop icons

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.

program_info/org.polymc.polymc.desktop.in

@@ -0,0 +1,12 @@
+[Desktop Entry]
+Version=1.0
+Name=PolyMC
+Comment=A custom launcher for Minecraft that allows you to easily manage multiple installations of Minecraft at once.
+Type=Application
+Terminal=false
+Exec=@Launcher_APP_BINARY_NAME@
+StartupNotify=true
+Icon=org.polymc.PolyMC
+PrefersNonDefaultGPU=true
+Categories=Game;
+Keywords=game;minecraft;launcher;