Provide an uninstall.sh file to fully remove Olympus

On Linux, the recommended method of installing Olympus is through the prebuilt zip (https://everestapi.github.io/ at `Installing Everest` for Linux) that includes the Olympus binary as well as a prebuilt LÖVE. There is also an install.sh which is recommended to run too. It doesn't do much apart from creating a .desktop entry for easier access to Olympus and setting the everest:// scheme handler.

However there is no way of undoing these changes unless you know where to look (/home/user/.local/share/applications and /home/user/.config/mimeapps.list) resulting in a half-uninstallation if only the source Olympus folder is deleted (as many people would do).

Accompanying install.sh should be uninstall.sh, a simple script that removes both the Olympus installation folder and the previously pointed leftovers. This saves time for unexperienced users having to look through their system to revert the changes.