Extending can be done from first node as well

The mExtendingFirst variable wasn't being reset.

The shape and position were not being set on the overlay object when
extending a polyline, so it only worked when those were still set from
the last created polyline.

Conflicts:
	src/tiled/editpolygontool.cpp

This way, the "Extend Polyline" action also works in the Tile Collision
Editor.

Also, it no longer appears in the Template view, because with the
current implementation it can't work there since the "Insert Polyline"
tool is not available there.

Don't switch to EditPolygonTool when user selected another tool. This
isn't what the user wants and it broke stuff.

There appears to be no difference between finishing or canceling the
extending of a polyline. In both cases we switch back to the "Edit
Polygons" tool and in both cases we should also make sure the object we
were extending is selected.

There shouldn't actually be a reason to touch the selection at all when
starting or stopping to extend a polyline, but currently the selection
outline does not update as expected while extending a polyline.