Optional polygon 2D editor single click editing

Following the discussion in https://github.com/godotengine/godot/pull/12740, this makes the whole new single click editor introduced in #10618 optional and it's no longer the default. The default is now the pre-10618-state of things with the additional ability to add and remove point using double clicking.

If single click editing is enabled in the editor options, it is now improved by adding a new one-click-delete (i.e. just click once on point to delete) that is the logical conclusion of #10618.

In both modes, the "delete point" button is now removed, as it's not needed in either mode.