text: Title: fails to find .desktop file to create autostart .desktop (autokey should search `$XDG_DATA_DIRS`)
  Body: ## Classification:
  
  Bug
  
  ## Reproducibility:
  
  Always
  
  ## Version
  
  AutoKey version: 0.95.10
  
  Used GUI (Gtk, Qt, or both): GTK (but shouldn't depend on GUI)
  
  If the problem is known to be present in more than one version, please list all of those.
  
  Installed via: self-packaged (built from source)
  
  Linux Distribution: GNU Guix
  
  ## Summary
  
  When autokey(-gtk) looks to create the autostart .desktop file, it looks to find the application desktop file, but only in some set locations (`/usr/share/applications` besides `$XDG_DATA_HOME`). Rather it should look in `$XDG_DATA_DIRS` for application desktop files to be more general. For example, this fails in distros like GNU Guix which do not use `/usr/share/applications`, but do properly set `$XDG_DATA_DIRS` as search paths for things like .desktop files.
  
  ## Steps to Reproduce (if applicable)
  
  - Try to enable autostart in the settings dialog
  
  ## Expected Results
  
  - The .desktop file should be found and an autostart entry created
  
  ## Actual Results
  
  - Instead, this happens. :(
  - output:
  
  ```
  2021-09-07 12:18:17,567 INFO - config-manager - Save autostart settings: AutostartSettings(desktop_file_name='autokey-gtk.desktop', switch_show_configure=False)
  2021-09-07 12:18:17,568 ERROR - config-manager - Failed to find a usable .desktop file! Unable to find: autokey-gtk.desktop
  Traceback (most recent call last):
    File "/gnu/store/khp...-python-autokey-0.95.10/lib/python3.8/site-packages/autokey/configmanager.py", line 209, in _create_autostart_entry
      source_desktop_file = get_source_desktop_file(autostart_data.desktop_file_name)
    File "/gnu/store/khp...-python-autokey-0.95.10/lib/python3.8/site-packages/autokey/configmanager.py", line 251, in get_source_desktop_file
      raise FileNotFoundError("Desktop file for autokey could not be found. Searched paths: {}".format(possible_paths))
  FileNotFoundError: Desktop file for autokey could not be found. Searched paths: (PosixPath('/home/podiki/.local/share/applications'), PosixPath('/usr/share/applications'), PosixPath('/gnu/store/khp...-python-autokey-0.95.10/lib/python3.8/config'))
  ```
  
  But, `autokey-gtk.desktop` is found in `$XDG_DATA_DIRS` (in this case in a profile directory, this is a Guix thing, but autokey should use XDG specs). This can be verified through using launchers like rofi, or with xdg tools.
  
  ## Notes
  
  I have not tried to patch this, but the relevant code is at https://github.com/autokey/autokey/blob/4b87b32452cd41d2b98341db220e99cf4f9e34d6/lib/autokey/configmanager.py#L234 Here, autokey should search `$XDG_DATA_DIRS` for the autokey .desktop file.