Some major bug fix and added experimental collider.

Added
1. Choice of adding experimental colliders with the tiles.
Bug Fixes:-
1. Fixed bug which cause incorrect marker positions while changing the
PixelSize in the editor and setting different values in x and y field.
2. Fixed bug causing deleting or modifying tiles when 2 or more layer
prefab exist in the scene to delete or modify tiles of other layer
prefabs.
3. Fixed bug causing Undo all the delete function with one undo.

Removed:-
1. Undo from the editor window changes as it is creating unnecessary
undo calls.