Removed an unnecessary header from class_board_item.h.

Added KiCad-style modification methods (hover over an item and press a hot key, without selecting first).
Modification point is selected basing on the number of selected items.
Rotation angle setting (Preferences->General) is taken into account while rotating.

- pcbnew.InteractiveSelection.Single for selecting a single item
- pcbnew.InteractiveSelection.Clear for clearing the selection
Made SELECTION_TOOL::clearSelection private.

SELECTION_TOOL::SELECTION::Clear made private, as there was no chance to call it outside the SELECTION_TOOL class.