Various Improvements

1) You can now put a finished painting back onto the easel and continue painting it.
2) The player counts as an object, which it attempts getting a luaentity for (which doesn't exist), then it tries to access the .grid nil value.
3) The meta that said whether the easel was diggable was either a) set to true or b) checked. It was never set to false, so I removed it and actually checked the above node.
4) It still aims wrong, even with the correct eye height from https://github.com/minetest/minetest/blob/master/src/player.h#L121