finders/desktop: support Terminal=true applications

[TheTrueWhiteOwl]

This adds a new dependency, namely to xdg-terminal-exec. I decided to use this script as a dependency, since, despite it still only being a proposal, there is already quite widespread use of it (e.g. wofi, glib).

Since this is better than the current total lack of support it seems reasonable and it also avoids any issues caused by implementing a default list, which by its very nature will never be fully complete, or will go out of date, and will just lead to users requesting their terminal be added to the default list.

[vaxerski]

I don't like it. Why not get the default terminal manually?

[TheTrueWhiteOwl]

Yeah I did this as the quick fix, lazily, because I already had xdg-terminal-exec installed abd I had a couple of terminal apps that I really wanted to work.

We could implement a fixed list of terminal emulators that we check (kitty, alacritty, wezterm, foot, termite, gnome-terminal, weston-terminal, konsole) check for the existence of a TERMINAL environment variable to indicate the default terminal, and maybe only after fall back to xdg-terminal-exec?

Or maybe even a configuration option that takes priority over all mentioned optioms above?

I am already using this patch for my use case, so I won't implement anything until we make it clear which of the above mentioned proposals would be desirable.

[vaxerski]

a config option, then env, then fixed is ok to me