Tksheet

Fixed:
- [157](https://github.com/ragardner/tksheet/issues/157)
- Double click row height resize not taking into account index checkbox text

Changed:
- Some internal variables local to redrawing functions for clarity

Fixed:
- `delete_rows()`/`delete_columns()` incorrect row heights/column widths after use
- Tab key not seeing cell if out of sight
- Row height / column width resizing with mouse incorrect by 1 pixel
- Row index not extending if too short when changing a specific index
- Selected rows/columns border fg not displaying in cell border
- Various minor text placements
- Edit index/header prematurely resizing height/width

Improved:
- Significant performance improvements in redrawing table, especially when simply selecting cells
- All themes
- Can now drag and drop columns and rows with or without shift being held down, mouse cursor changes to hand when over selected

Changed:
- Dropdown box colors now use popup menu colors
- Checkboxes no longer have X inside, instead simply a smaller more distinct rectangle to improve redrawing performance

Fixed:
- [152](https://github.com/ragardner/tksheet/issues/152)

Fixed:
- `set_row_heights()`/`set_column_widths()` failing to set if iterable was empty
- `delete_rows()`/`delete_columns()` failing to delete row heights, column widths if arg is empty
- Edges of grid lines appearing when not meant to

Changed:
- Add `redraw` option for `change_theme()`

Fixed:
- Dropdown boxes in main table not opening in certain circumstances
- Scrollbars not having correct column/rowspan
- Error when moving header height
- Resizing arrows still showing up when hiding header/index if height/width resizing enabled

Added:
- `header` startup argument, same as `headers`

Changed:
- Increase default maximum undos from 20 to 30
- Removed dropdown box border, to get them back use `show_dropdown_borders = True` on startup or with `set_options()`
- Removed unnecessary variables and tidied `__init__` code

Fixed:
- [154](https://github.com/ragardner/tksheet/issues/154)
- `delete_column()` not working with hidden columns
- `insert_column()`/`insert_columns()` not working correctly with hidden columns

Added:
- `delete_columns()`, `delete_rows()`

Changed:
- `delete_column()`/`delete_row()` now use `delete_columns()`/`delete_rows()` internally
- `insert_column()` now uses `insert_columns()` internally