Merge lp://qastaging/~jwc/mixxx/features_trackloadoverride into lp://qastaging/~mixxxdevelopers/mixxx/trunk

Adds a preference option that allows the user to turn on or off the ability to load a track into a playing deck. By default, this feature is disabled (which is the current method of operation -- Mixxx won't let you load tracks into playing decks).

This does not enable the "Load into Deck x" context menu options. I believe that those are very dangerous to enable for a playing track because it's easy for your mouse to slip. Instead, it enables the ability to load into decks by pressing keyboard shortcuts ( [ and ] ) or through your controller.

This feature is especially useful for vinyl control users.