Add un-check all button to the tree-based move

[michitux]

The tree-based move manager should get a button/link to un-check all items so after moving a lot of selected items the selection can be cleared easily. Un-checking items automatically might not be desirable as possibly the user moved the items to the wrong location and wants to move them again.

[ryliejamesthomas]

This would be handy. Recently used the plugin to move a lot of pages, and got caught-out once because I didn't notice some things were still checked.