Add settings menu

This merge request would add a settings menu, accessible through the cog wheel at the top left of Hourglass™'s interface.

In addition to this, the option to change the directory Godot versions are installed to has been added (in the future we should add a way to migrate the versions from the old directory to the new one). This closes #3.

The option to keep Hourglass™ running after opening a project has also been added. This closes #8.