Part 1 of XKB support.

Part 1 of XKB support.

This change includes:
1. Adds hotkey for switching to previous engine (renaming the old unused "prev_engine" to previous_engine).
2. Renames next_engine hotkey to next_engine_in_menu.
3. Saves/loads current and previous global engine names to/from config.
4. Adds enable_by_default config to enable the input method by default.
5. Moves hotkey filter function from inputcontext.c into ibusimpl.c.