Show a "What's Here" box in sidebar when mouse hovers over a tile

This incorporates the input refactor PR #3766.

This is SDL only.

Shows an enhanced "Look Around" box below the status area, when the mouse hovers over a visible tile. Box goes away if a key is pressed, or if the mouse is moved off the terrain window. Only active if mouse cursor is not hidden. Works in both standard and narrow status views.

Since the there's plenty of room below the status area in the side bar, this box can grow to show all the items visible on a tile (within reason), unlike the default "Look Around" which only shows you one item.