Cows remain on invalid square with no feedback

[faucomte97]

Describe the bug
IMPORTANT This task should be completed once task #1672 has been done. This is because this task assumes that cows can be placed anywhere on the map except start and house squares.

When a user tries to place a cow on an invalid square in the level editor (start or house squares), the cow just stays there, even though it still hasn't been registered as properly "placed" in the level.

To Reproduce
Steps to reproduce the behaviour:

1. Go to the level editor.
2. Create a road, and mark the start and add one or many houses.
3. Go to the Scenery tab, and drag a cow onto the map.
4. Try to place the cow on either the start square or any of the house squares.
5. Notice that the cow just stays there, even when it's in invalid square. There is no feedback for the user.

Expected behaviour
When trying to place the cow on an invalid square, the cow should snap to the closest valid square.
Edge case: in the event where there are no remaining valid squares, the cow should just be removed from the map.

Screenshots

The cow on the road is on a valid square, you can tell this by the light grey highlight on the square.
The cow on the start square is still there, despite the fact that it's an invalid square.