This "ui-active" property should be set to FALSE for new incoming calls and set
to TRUE after a short delay of 20ms. This property is used to delay showing the
call display and starting to ring.
This delay gives DBus clients like
phosh-antispam (https://gitlab.com/kop316/phosh-antispam)
some time to make a decision whether a call should be hung up or not and defer
starting to ring and showing the display only afterwards.
We need a proxy object because implementing the CuiCall interface in the
CallsCall base class has it's own set of problems (f.e. the "state" property
would have a type collision CallsCallState vs CuiCallState).
