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.
Use uppercase mnemonics in translations.

Issue: https://github.com/choehwanjin/ibus-hangul/issues/19
Remove deprecated Encoding key from the FDO desktop entry
Add Shift+space to the default toggle key list
Merge pull request #21 from changwoo/master

Remove deprecated Encoding key from the FDO desktop entry
Merge pull request #1 from changwoo/hangul-toggle

Add Shift+space to the default toggle key list
Add Hangul toggle key setup
Update hangul mode property on toggle key
Change property label: "Hangul lock" -> "Hangul mode"
Merge pull request #2 from changwoo/hangul-toggle

Hangul toggle improvements
Merge pull request #18 from ueno/hangul-toggle

Allow hangul mode to be toggled
Rename "hangul_mode" property to "InputMode".

And Add symbol to InputMode property.
This will show InputMode status on gnome-shell.
Add "initial_input_mode" config item.

If initial_input_mode is "hangul", then hangul engine will start
in hangul mode, otherwise in latin mode.
I've changed hangul_mode to more general name, input_mode.

I will add UI for this config item later.
Use "initial_input_mode" config item on startup

When the "initial_input_mode" config item is changed,
there is no need to change the current input mode.
"initial_input_mode" item should be applied on staring routine.
setup: Use GtkBox instead of GtkVBox, GtkHBox
Add "Start in hangul mode" option

If this widget is checked, "initial_input_mode" config will be set
as "hangul".

This widget is not match to the config item "initial_input_mode".
But I think "Start in hangul mode" check box is easier to use
than "initial_input_mode" combo box.
Correct spacings

See https://wiki.gnome.org/Design/HIG/Layout
Merge pull request #24 from changwoo/setup-ui-cleanup

setup: Add spacing and clean ups
Add off key feature.

On Escape key event, the input mode will be changed to latin mode.
This will help vi* users.
Add on key feature