Add support for xdg-terminal-exec for handling desktop applications using 'Terminal=true'

This is implementing the suggestion in #338 (comment 1076172)

I also took the opportunity to refactor the prepend_terminal_to_vector functions to:

• simplify the logic
• make the list of supported terminals more clear

/cc @pwithnall as this was his suggestion.

Fixes: #338 (closed)