The app.desktop file shipped with the default build presumes the install
location of the binary. Let the OS determine the executable through it's
normal PATH variables.
Inspired by #413.
This modifies the .desktop files so that it's possible to right-click on
the Ghostty icon in a dock and get the "New window" action.
Apparently this is how you implement that in GTK applications. In order
for us to also get "New tab", we'd have to implement a CLI flag, I
think.
