Make keymap revertable

[Grannath]

The plugin adds a keymap Emacs+ that is a modified version of the OOTB Emacs keymap. This unfortunately means I cannot revert to the pseudo-OOTB keymap Emacs+.

I recently had some issues, because the plugin got deactivated (no idea why). This messed with the keymap, and the only way to get it back was to remove the plugin, delete the keymap, reinstall the plugin.

Ideally, for me, the Emacs+ keymap is treated as an OOTB next to Emacs. Second-best option is a new menu entry for the plugin to restore the included keymap.