gdesktopappinfo: Fail early if trying to launch an invalid executable

GDesktopAppInfo never failed in the most simple of the cases: when a
desktop file or a command line app info was pointing to an invalid
executable (for the context).

The reason for this is that we're launching all the programs using
gio-launch-desktop which will always exist in a sane GLib installation,
and thus our call to execvp won't ever fail on failure.

This was partially mitigated by not allowing to create a desktop app
icon using a non-existent executable (even if not fully correctly) but
still did not work in case a custom PATH was provided in the launch
context.

To avoid this, use g_find_program_for_path() to find early if a program
that we're about to launch is available, and if it's not the case return
the same error that g_spawn_async_with_fds() would throw in such cases.

While this is slowing a bit our preparation phase, would avoid to leave
to the exec function the job to find where our program is.

Add tests simulating this behavior.