Animation: KisLayerFilterWidget & Text-based Filtering

Added new KisLayerFilterWidget.

Eoin and I have made a new widget for filtering the layer tree by name and/or color label. We've also done a little bit of refactoring where possible for the sake of reasonable code reuse. WIP, as there may be a couple of remaining bugs.

The ability to filter layers by name was requested by Bollebib.

Test Plan

Open a file with a lot of layers, try opening the new KisLayerFilterWidget by clicking on the small square button on the upper-right corner of the Layers Docker. Using this widget you should be able to filter layers by name or by color-label. Please help test!

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.