Add Support for buildapp for Linux

[Calebh101]

I added a script called buildapp-linux.py in Scripts that generates an executable for Linux users, similar to buildapp-select.py, officially supporting Debian-based distros. The script basically generates a payload, a main script, and an installer script. The main script handles things like extracting itself, handling temporary files/directories, and managing data. buildapp-linux also compiles a generated C file using gcc that extracts the main script from itself, then runs it. The installer script adds ProperTree to ~/.local/bin and creating a desktop entry.

Even though it doesn't have -select in the title (mainly for consistency), the desktop entry specifies the executable as supporting text/xml and application/x-plist, which is associated with .plists in Linux, thus providing -select capabilities.

I also added two other new files: Scripts/linux-app.c and Scripts/icon.png. linux-app.c is the file containing the base source code for generating the executable, and icon.png is a 256x256 image extracted out of shortcut.icns that is used for the icon for the desktop entry. (I did this because Linux can't easily read .icns files without third-party tools.) Please note, when you change the icon, I'd recommend also changing icon.png, as that's used as the application icon.

I also attached a link the documentation I wrote for buildapp-linux.py that you can add manually to the end of README.md or add it as a separate in the repository. (Or you can write your own if you want to.) I made it with a similar in format as the section describing buildapp-select.py's usage. The documentation I wrote can be reformatted or edited if you'd like. ProperTree-buildapp-linux-Usage.md.