Check for gnome-terminal-emulator and x-terminal-emulator

When opening .desktop files set with "Terminal=true", the terminal is chosen from a hardcoded list. For users of the terminal emulators in the list, the terminal chosen was suitable, but for those without any terminal listed it's falling back to xterm. With this commit, gnome-terminal-emulator and x-terminal-emulator are checked after gnome-terminal and before the others. For users of gnome-terminal and from the other terminal emulators in the list it should have no changes, but for users of other terminals, as tilix, and on a system that uses gnome-terminal-emulator or x-terminal-emulator, the terminal set in this two options will be honored, instead of opening xterm.