Added functionality to switch selected block using the buttons. Just added a few child buttons to LevelEditorMenu. Note that directly adding a button to a
buttonlist of either a menu or a collapsablebutton directly using new will lead to memory leaks, because they are never deleted. Also removed the need for
passing a string of the filename of the selected block to a button by checking the  Button* provided at a callback against the buttons of the menu tou know
which button it is(and when we know which button it is, we also know which block should be made active.) I didn't make a nice wrapper yet though, since we're
not sure what way of loading blocks from a file is going to be used yet.