cmake: Reorganize package finding

Use COMPONENTS to find required Qt5 modules in one go.
Don't look for QtWebKit if we already have QtWebEngine.
Look for Sonnet explicitly even when using KDE Frameworks, because
we're using its config widget and shouldn't rely on transitivity
here.
Only look for LDAP when building the core (or mono client).
Add QUIET everywhere to avoid spammy CMake warnings for optional
packages.
Add some diagnostic output indicating the versions of Qt and KF5.