tiles: Set up proper quick tile tree hierarchy
When a window is moved between two quick tiles, Tile::addWindow() will evacuate the window from the previous tile in the corresponding tree. In order to achieve that, the addWindow() function finds the root tile and then walks the tree.
Currently, quick tiles have no proper tile tree hierarchy setup, so the rootTile() getter is going to fail to find the correct root tile. This would eventually result in Window::requestedTile() getting stuck and not being able to drag the quick tiled window.