[mce] Add missing methods and signals. MER#1854

Some methods and signals belonging to mce D-Bus interface are
not declared in mce-dev package. And what is declared is not
grouped in a manner that would make the context of constants
clear.

Add missing method/signal/argument constants.

Group constants by context.

Add references from methods/signals to relevant argument
constants.

Do not refer to glib types when explaining a D-Bus interfaces.

Where needed update the explanatory comments.

Signed-off-by: Simo Piiroinen <simo.piiroinen@jollamobile.com>