Devices designed with Android in mind can have menu/home/back
buttons in front face. In some devices these buttons are
practically invisible unless the underlying button backlight is
turned on.

Add D-Bus interface that can used by applications and/or
lipstick for requesting and tracking button backlight state.

Implement policy that turns button backlight on when:
- Related sysfs controls have been defined in mce configuration
- At least one D-Bus client has requested backlight
- Display is powered up
- Lockscreen is not active
Signed-off-by: Simo Piiroinen <simo.piiroinen@jollamobile.com>