Psg-reskinner

This version fixes issue 11...
Thanks to arminposchmann for reporting: in previous versions, using the line-divider in menus would break Reskinner. No longer a problem.

...and brings _easing functions_! (CSS lovers rejoice.)

Animated reskins can now utilize one of _twenty-five_ built in easing functions of many types, including `SINE`, `CUBIC`, `QUART`, `CIRC`, `BOUNCE` and more to make transitions really pop, courtesy of ai's gifted work with [easings.net](https://github.com/ai/easings.net). If the 25 aren't enough, you can provide your own; the new `easing_function` parameter on the `animated_reskin` function takes any callable that takes the current animation timing as a `float` between 1 and 0, and returns the desired animation progress, also as a `float` between 1 and 0.

Once again, please give this project a star if you find it useful. Enjoy.

This release brings major improvements and minor breaking changes to Reskinner, mainly:

- The `exempt_element_keys`, `target_element_keys` and `honor_previous` parameters have been removed from the `reskin` and `animated_reskin` functions. An `element_filter` parameter has been added instead, which takes any callable that takes individual PySimpleGUI elements and is expected to return either `True` or `False`, and if given, only elements that cause it to return `True` will be reskinned. This enables more complex element selection, like filtering or selecting even elements with no key.
- `COLOR_SYSTEM_DEFAULT` shouldn't be a problem anymore; Reskinner now makes use of actual default elements behind the scenes to obtain the right colors.
- The interpolation logic has been reworked; it no longer does any weird colordict manipulation.
- The color selection logic has been reworked as well, with caching as an added feature.

Future releases will focus more on performance and optimization, and possibly the return of the `honor_previous` parameter.

In the meantime, enjoy.

This version mainly addresses the issue with Default PySimpleGUI themes, specifically the issue with COLOR_SYSTEM_DEFAULT (or 1234567890), among other fixes and improvements.

Enjoy.

This version fixes the instability observed after 1000 reskins, or 100 animated reskins, as first noted in issue 1 . It also removes the dependency on `TITLEBAR_TEXT_KEY` and other similar PySimpleGUI constants, instead finding the titlebar elements by introspection (fixing the side effect that caused issue 6 ).

Several other fixes have been made, all in all making performance more stable. Enjoy.