Allow hangul mode to be toggled

Since 1.5 of IBus, IME's enable/disable feature has gone, and one can
no longer quickly turn on/off IME status by keyboard (although he can
switch between an XKB layout and the Hangul IME with Super-Space, it
is known to be too slow/inconvenient).  So, some IMEs have already
started providing enable/disable feature by themselves.  See:
https://fedorahosted.org/i18n/ticket/26

This patch makes hangul_mode (currently not in use) to be switchable.
This is implemented in a similar manner as hanja_mode, except that I
renamed HanjaKeyList into more generic HotkeyList.