Allow setting collisions for multiple tiles at once

Often a large number of tiles on a sheet will have exactly the same collision boundaries and properties (basic walls, platforms, etc). It would save a lot of time if collisions could be applied to multiple selected tiles in the Tile Collision Editor.

This would likely include a popup to warn the user that individual tile collision assignments would be overwritten.

Alternatively, maybe the default behavior could be to "add" the new collision to to the tiles rather than overwriting them, along with an option to clear the previous assignments from the selected tiles. This could work well if the dialog displayed all of the selected tile's collisions superimposed on each other (uneditable unless they all matched). Not sure about the usefulness versus complexity of this approach.

As discussed in this issue:
https://github.com/bjorn/tiled/issues/5#issuecomment-234915789

Start in the Tile Collision Editor and read the discussion linked from issue #5. Clarify whether applying collisions to multiple selected tiles should overwrite or add to existing assignments, and define the warning or clearing behavior. Done means a consistent multi-tile workflow with the selected tiles' collision assignments handled as specified.