modernize: Replace most remaining old-style connects by PMF ones

modernize: Replace most remaining old-style connects by PMF ones

Manually replace old-style connects (using the SIGNAL/SLOT macros)
by the much more efficient and typesafe pointer-to-member-function-
based ones.

This fixes the cases where clazy could not auto-migrate to the new
syntax for a variety of reasons. In most of the cases, we need to
remove overloads or explicitly select the desired one, because the
plain syntax cannot deal with overloads. Another issue is trying
to connect to signals in a baseclass that are only declared in a
derived class (which works with the old-style runtime connection
handling, but obviously no longer with the compile-time version).

Also do some selected cleanups in places.