Settings: Volume panel background color && transparency [1/2]

Allow user to specify background color and transparency level
    for volume panel.

PS: make "TRDS default" summary value a resource so
        it can be translated.

Change-Id: Ic788b4ccf1826075a600e53bc6a7c3c8bda50685

Conflicts:
	res/values-fr/slim_strings.xml
	res/values-it/slim_strings.xml