data: Add X-Purism-FormFactor to desktop file

This allows shells like phosh to filter on form factor so
the app list isn't cluttered with applications that don't
fit the screen when not docked.

Reference: https://source.puri.sm/Librem5/phosh/-/merge_requests/639

data/org.postmarketos.Megapixels.desktop

@@ -5,3 +5,4 @@ Terminal=false
 Type=Application
 Categories=GTK;
 Icon=org.postmarketos.Megapixels
+X-Purism-FormFactor=Workstation;Mobile;