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.desktop

@@ -0,0 +1,19 @@
+[Desktop Entry]
+Name=gLabels Label Designer 4
+Comment=Create labels and business cards
+Comment[de]=Entwurf von Etiketten und Visitenkarten
+Comment[es]=Crear etiquetas y tarjetas de visita
+Comment[fr]=Créer des étiquettes et des cartes de visite
+Comment[it]=Crea etichette e biglietti da visita
+Comment[pl]=Twórz etykiety i wizytówki
+Comment[pt]=Crie etiquetas e cartões de visita
+Comment[pt_BR]=Crie etiquetas e cartões de visita
+Comment[uk]=Створюйте етикетки та візитки
+Keywords=label;card;print;office;barcode;
+Exec=glabels-qt %F
+Icon=glabels
+Terminal=false
+Type=Application
+Categories=Office;
+StartupNotify=true
+MimeType=application/x-glabels;