Implement proper package system

[p-e-w]

Taking a page out of Sublime Text's great book, Final Term should have its own Packages directory in .config with the property that any Final Term extension file (.ftcolors, .fttheme, .ftkeys, .ftcommands, .ftmenu, ...) in a subfolder of that directory automatically gets recognized and loaded on startup.

This would allow people to package all customizations forming a plugin into, say, a GitHub repo, which can then be "installed" with a simple git clone, and pave the way for a built-in extension management system.

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