To improve the readability of the starting wrapper script let's split
off all the helper stuff like local functions, static strings and some
checks. The plain wrapper script for /u/b/thunderbird get consolidated
and compacted by this.

Also adding the Mate DE to the detected environments we can run on, as
suggested by #855286.
Adding the 'mate' string to the zenity call if detected. We still need
to add some sanity checks if zenity or kdialog available.

Closes: #855286