UI system rework

[moonburnt]

• Rework UI system to use Nodes under the hood.
• Implement a proper alignment system, similar to the one adopted in Qt5.

[moonburnt]

Work on this feature happens together with #6 in ui_rework branch.

What s done:

• Implemented a basic alignment system.
• Reworked Label -> TextNode.
• Reworked FPS counter to be based on Node.
• Removed ButtonsStorage as obsolete.

What s left:

• Rework buttons.
• Rework bars.
• Rework checkboxes.
• Consider looking into MVC-like behavior for UI element's controls (separate controller from interactive elements. Different controllers may then be used, depending on input method).