fixed 0 flags to be -1 (hold everything, while 0 does nothing), added flags description to docstrings

+ Item.selected.setter
+ Item.set_info_value()
+ Take.set_info_value()
+ Take.source.setter
+ Take.start_offset.setter

~ fix typo in Take.add_event (take.pyi)

I'm not sure exactly what I've done. But it brang stability to my script

Previously, marker index was enum index to be used in EnumProjectMarkers2, but now the additional member Marker.enum_index : int is introduced.
Marker.index now responds for the user-ID, that could be changed bu shift+click in the arrange-view

class EnvelopePoint
Envelope methods:
* `get_point(self, index:int) -> EnvelopePoint`
* `insert_point(self, point: EnvelopePoint, sort: bool = True) -> bool:`
* `set_point(self, index: int, value: EnvelopePoint, sort: bool = True) -> bool:`
* `sort_points(self) -> None:`

Since `import reapy` line is needed only for keeping the version, I've made it directly by file-parcing. Now there is no strict need to import reapy before setuptools make its work.

As within the same commit relative to markers I've divided `region.index` (which now is user-index from edit popup) and `region.enum_index` (which is what was index previously).

…res#112

I've added it in this branch as the branch already has `Item.is_selected.setter` property