gsd: Fix keyboard backlight toggle state machines

Some laptops like ASUS UX550GE has a hotkey (Fn+F7) for keyboard
backlight toggle.  In this UX550GE, the hotkey not only switches on/off
the keyboard backlight, but also increments the level of brightness
from 1 to max 3.  In this case, it will trigger the keyboard backlight
StepUp and Toggle events and execute the handle_method_call_keyboard
function of gsd_power_manager.c in GNOME settings daemon when the hotkey
is pressed.

The ideal scenario of pressing hotkey should trigger the events like:

.. StepUp -> Toggle -> StepUp -> StepUp -> StepUp -> Toggle -> StepUp ..

However, we noticed the actual processes are:

.. StepUp -> Toggle -> Toggle -> StepUp -> StepUp -> StepUp -> Toggle ..

It triggers Toggle event two times between StepUp events.

We check the upower_kbd_toggle function called in
handle_method_call_keyboard when the event is Toggle, and find out it
checks only the state of old brightness without current brightness.
That misses some other states changing.

So, here is the list of the Toggle Method's state machines:
1. brightness_now: 0, brightness_old: 1 ~ max level of brightness
        -> toggle to old brightness
2. brightness_now: 0, brightness_old: -1 or 0
        -> toggle to max brightness
3. brightness_now: 1 ~ max level of brightness, brightness_old: all
        -> toggle to 0 brightness

This commit fixes and implements the state machines.  Closes #41

Signed-off-by: Jian-Hong Pan <jian-hong@endlessm.com>
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