☄ Move categories & entities via drag&drop

[bitbrain]

Introduce a mechanism that allows for drag&dropping categories & entities in the tree in order to move them around and effectively re-assigning parent categories to them. As a result, some of its properties change, so doing so should prompt a popup to ask the user if this is really something they want to do (only if properties actually would change).

This should also allow for reorganising your entities & categories, as well as selecting multiple entities/categories at once that can be dragged and dropped.