Key actions: added support for custom actions
Previous functionality was limited by only 2 custom
actions "Launch custom app 1" and "Launch custom app 2"
which needed to be set using separate preferences which was 
confusing for the most of the users.

This major rework improves user experience and brings more flexibility
by allowing to choose "Custom action" entry from the list of available
key actions.
When "Custom action" is selected, hidden app picker preference dialog
will show
for user to choose a custom action. Summary description of key action
preference
reflects the description of custom action that was chosen.
This removes limit of custom actions and brings more transparency to the
whole process.

All preferences that use key actions preference list have been unified
to simplify
their handling (Pie longpress actions, custom navbar key actions).