Revert "Reworked the UI to make it look more modern."

I've reverted this branch, because with new gui, the program lost a lot of
its features, like builtin help. Older gui is uglier, but certainly more
functional.

New gui is still saved in 'nice_gui' branch.

This reverts commit 24a8a3be92ed31c22c68fa681c3b64f24a027b7c.