connect groupbox toggles to change signal

Summary:
the bridge, ppp, and gsm settings contain qgroupboxes which previously
were not tracked

e.g. the STP box toggle on bridges didn't enable the apply button

since checkable qgroupboxes are a fairly generic concept, let's connect a
generic watch rule

Test Plan: toggling STP in bridge settings actually enables apply button

Reviewers: jgrulich!

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D25368