Tksheet

Fixed:
- Cell editor right click not working
- Cell editor select all not working

Added:
- Some methods to `Sheet` objects, see [documentation](https://github.com/ragardner/tksheet/wiki/Version-6#sheet-methods) for more information.
- __bool__
- __len__
- __iter__
- __reversed__
- __contains__

Fixed:
- Incorrect row being targeted with hidden rows and text editor newline binding (potential error)

Fixed:
- [203](https://github.com/ragardner/tksheet/issues/203)

Fixed:
- [202](https://github.com/ragardner/tksheet/issues/202)

Fixed:
- [201](https://github.com/ragardner/tksheet/issues/201)
- The ends of grid lines were incorrectly displaying connections with one another when only showing horizontal or vertical grid
- When a cell dropdown and a row checkbox were in the same cell both would be drawn but only one would function, this has been changed to give dropdown boxes priority
- Index text overlapping checkbox when alignment is `"right"`/`"e"`/`"east"` and index is not wide enough

Added:
- [200](https://github.com/ragardner/tksheet/issues/200)

Fixed:
- Error when zooming or using `see()` with empty table
- Add initialization option `zoom` which is an `int` representing a percentage of the font size, `100` being no change

Removed:
- Some unnecessary internal variables