Use consistently Q_SIGNALS/Q_SLOTS (and set QT_NO_SIGNALS_SLOTS_KEYWORDS)

Summary:
Currently there is a mix of both signals/slots and Q_SIGNALS/Q_SLOTS.
Given Marble is also promoted as library, it seems to make sense to
standardize on Q_SIGNALS/Q_SLOTS, and for consistency do that project-wide,
not just for the lib API.
Adding QT_NO_SIGNALS_SLOTS_KEYWORDS should prevent any regressions here.

Test Plan:
Still builds (WITH_KDE=ON & BUILD_MARBLE_EXAMPLES=ON) and marble app still
works.

Reviewers: rahn, nienhueser

Reviewed By: nienhueser

Differential Revision: https://phabricator.kde.org/D1608