support adaptive full height viewport controls

[axelpale]

Consider a viewport control that acts as a sidebar.

Currently Viewport reacts to 'resize' event that originates from ResizeCapturer. Upon resize, Viewport calls ViewportControls:adapt that maintains the relative positions of existing viewport controls.

A sidebar control has full height. This height does not adapt to viewport size change.

Possible solutions:

• allow controls to have percentage width/height (does not work well for absolutely positioned elements)
• keep relative size of the controls constant (does not work for buttons and other constant-sized controls)
• add options how controls should adapt: e.g. boolean absoluteWidth, absoluteHeight, absoluteX, absoluteY.
• create dedicated class for sidebar: e.g. SidebarControl