Detect if UI element is above a block when clicking in the block list

[TheRealCatherine]

In Edit Mode on the right hand side there is a list of block types that can be placed ,but if a block is behind where the user clicks it will select the block (and maybe also place the new one too) due to a click handler in AbstractBlock. The click handler is so the user can easily select blocks using the mouse but it doesn't take into account UI elements.