Tksheet

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

Fixed:
- [153](https://github.com/ragardner/tksheet/issues/153)

Changed:
- Adjust dropdown arrow sizes for more consistency for varying fonts

Fixed:
- Issues and possible errors with dropdowns/checkboxes/cell edits
- `delete_dropdown()`/`delete_checkbox()` issues

Changed:
- Deprecated external functions `create_text_editor()`/`get_text_editor_value()`/`bind_text_editor_set()` as they no longer worked both externally and internally, use `open_cell()` instead
- Renamed internal function `get_text_editor_value()` to `close_text_editor()`

Improved:
- Slightly boost performance if there are many cells onscreen and gridlines are showing
- You can now use the scroll wheel in the header to vertically scroll if there are multiple lines in the column headers
- Improvements to text editor
- Text now draws slightly closer to cell edges in certain scenarios
- Improved visibility of dropdown box against sheet background
- Improved dropdown window height
- black theme selected cells border color
- light green theme selected cells background color