Add Flatpak permissions

[ntninja]

Nice to be using an Electrum fork with native UI again!

Flatpak needs to following extra permissions for theme integration (also used by org.kde.Runtime):

• --filesystem="xdg-config/kdeglobals:ro"
• --filesystem="xdg-data/themes:ro"
• --filesystem="xdg-config/icons:ro"

Flatpak also needs the following extra permission for the tray icon:

• --talk-name=org.kde.StatusNotifierWatcher