allow point-and-click MIDI learning of Hotcue buttons

[ronso0]

Hotcue buttons can not be mapped with the MIDI Learning Wizard with point-and-click, because the widget connections of WHotcueButton are created by the class itself, not by LegacySkinParser::setupConnections, so the controls are not available in the ControllerLearningEventFilter (actually only activate and enabled are controls, remove and the context menu are handled in mousePressEvent)

Note: this is not a big deal since the controls are available in the controls menu, though to accelerate mapping point-and-click would be nice.

[ronso0]

This quick patch adds _activate to the learning filter
MIDI-learning-allow-learning-HotCue-button-with-poin.patch.txt broken
Pick it up if you like, I don't have any motivation to get this to work for _clear. Maybe that can be accopmplished with a helper in WHotcueButton, idk