Preparation for flatpack support (#255)

- Updates to project and desktop metadata in preparation for flatpak packaging
- These updates are independent of flatpack
- This commit unmingles the actual flatpack packaging from #255

data/org.glabels.glabels-qt.mime.xml

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<mime-info xmlns="http://www.freedesktop.org/standards/shared-mime-info">
+   <mime-type type="application/x-glabels">
+     <comment>gLabels Project File</comment>
+     <sub-class-of type="application/xml"/>
+     <icon name="x-glabels-project"/>
+     <glob pattern="*.glabels"/>
+   </mime-type>
+</mime-info>