Changing UI elements to have a single common class

[CRImier]

Currently, there are a lot of UI elements which could share primitives - keymap handling, for one. It's going to be useful for further splitting UI elements into smaller classes - allowing to share more and more primitives. I'm going to split common functionality into a single base class and then see where it can go.

[CRImier]

UPD - this will not, for now, touch Printer elements - which are functions.

[CRImier]

Changes are now in devel branch. They allow to write UI elements much quicker (using the BaseUIElement class) and assign keymap entries more easily (passing strings as references to in-class methods that don't need the UI element to go into background, and passing functions where the UI element has to go into background).

• [ ] Needs tests for what isn't yet covered by tests - maybe cover the BaseUIElement itself (even though it's pretty well-covered by existing tests on its subclasses)
• [x] Needs manual testing for each changed UI element - just to make sure
• [ ] Needs explanations and examples on how the BaseUIElement could be used to develop your own UI elements
• [x] Needs documentation for the new keymap assignment method - so that developers can use it successfully

[CRImier]

I consider this finished - pulling the remaining TODOs out into separate issues.