Make unblank and unlock via double tap work again

Legacy option to allow bypassing of lockscreen with doubletap was
broken.

Make it work again.

Also do not use raw numeric values to evaluate doubletap_gesture_policy.

[mce] Make unblank and unlock via double tap work again
Add helper functions for setting up configuration change tracking

Having helper functions for both reading initial values and installing
change notifiers allows builtin-gconf value tracking setup to be more
compact and readable.
Rewrite powerkey handler to allow more flexible configuration

Taking double powerkey presses in use caused unconditional delay for
single press handling. The actions that could be configured to be taken
were not be differentiated based on display state, which made it
impossible do one thing from display off and another one from display on.
And since static configuration was used, changes did not take effect
without restarting mce.

Make possible actions as small and simple as possible. Allow combining
them freely to choose actions taken. Use separate action combinations
depending on whether display is on or off. Execute common parts of
single and double press actions immediately when 1st powerkey press
is released.

Allow more flexibility for using custom dbus actions by allowing user
configured method call to be made instead of signal broadcast.

Use dynamic runtime changeable settings instead of static configuration
from ini-files and add options to mcetool for changing all of them.

Use the new functionality to:
- apply device lock when double press is made from display on
- unblank and hide lockscreen when double press is made from display off

[mce] Rewrite powerkey handler to allow more flexible configuration. Fixes JB#23653