Better tag color cache management

See the code near this line: https://github.com/ForNeVeR/Todosaurus/blob/908e172b753c0e59edec0e3b99d1131e34ce7c9d/core/src/main/kotlin/me/fornever/todosaurus/core/ui/controls/tagList/TagColors.kt#L19

Also, look for the number of this issue in the project code base.

We have the following problems:
- It seems that the color of `UIUtil.getPanelBackground()` is not updated after changing the theme in the IDE, so, we re-calculate the colors.
- We need to calculate colors for light and dark themes exactly once and not re-calculate.
- Expand the cache so that it stores colors for dark and light themes separately (currently, the cache only stores colors for one theme).