Add standard categories to desktop file

The Photography category is probably an obvious one[1].
For the main category, I think Graphics fits better than Office

[1] https://specifications.freedesktop.org/menu-spec/latest/apas02.html

data/org.postmarketos.Megapixels.desktop

@@ -3,7 +3,7 @@ Name=Megapixels
 Exec=megapixels
 Terminal=false
 Type=Application
-Categories=GTK;
+Categories=GTK;Photography;Graphics;
 Icon=org.postmarketos.Megapixels
 X-Purism-FormFactor=Workstation;Mobile;
 StartupNotify=true