Custom KeePassXC desktop launcher

KeePassXC (1) supports GTK3 out-of-the-box, but unfortunately doesn't
adapt to the current system theme and icon them making the UI looking
really unpleasant and outdated.
To use a modern style the app is now started with the
`QT_QPA_PLATFORMTHEME` environment variable (2) set to `gtk2` to ensure the
app respects the Qt5 style overrides (3).

References:
  (1) https://keepassxc.org
  (2) https://wiki.archlinux.org/index.php/qt#All_Qt5-based_applications_fail_to_run_after_Qt5_update
  (3) keepassxreboot/keepassxc#1931

Resolves GH-204

snowblocks/xdg/applications/org.keepassxc.KeePassXC.desktop

@@ -0,0 +1,9 @@
+[Desktop Entry]
+Name=KeePassXC
+Exec=env QT_QPA_PLATFORMTHEME=gtk2 keepassxc %f
+Icon=keepassxc
+StartupWMClass=keepassxc
+Terminal=false
+Type=Application
+Version=1.0
+MimeType=application/x-keepass2;