Move selection manipulator mode methods to RadiantSelectionSystem (WIP)
Move more selection mode methods into RadiantSelectionSystem
Drag mode toolbar icon is now activated on startup.
Move deselect command code to RadiantSelectionSystem

plus code cleanup
Move command registration calls to selection::algorithm
More command refactoring.
Fix an uninitialised pointer.
Move more command stuff around.
Remove the legacy mainframe_old file pair.
Merge code to avoid code duplication