Use Gio to open files, fix "no default app" issue

Fixes issues where files with no default application set would error on right-click and do nothing on double-click.

When right-clicking these files, now hides the "Open" menu item and instead shows "Open With..." so app can be selected.

When double-clicking these files, now opens the "Open With..." dialog.

In the process of fixing the double-click issue, switched from opening files with xdg-open to opening files with Gio, which should now better/exactly match the right-click menu.