modernize: Migrate action-related things to PMF connects

In our quest to get rid of the deprecated use of slots, refactor
Action and ActionCollection to require pointer-to-member-function
connects. Ensure nobody accidentally still uses SLOT through a static
assertion.

Add a convenience function to ActionCollection for adding lists of
actions. While we're at it, also rework ActionCollection's optional
KDE Frameworks support to simplify things.

Adapt the rest of the code accordingly.