* Implemented ButtonGroupManager.
  It is responsible for positioning of buttons, any animations
  during switching, etc.
* Added MaterialButtonLookAndFeel class which implements
  flat button look and feel following Google Material Design Guidelines.

* Implemented sliders' material lookAndFeel;
* Material lookAndFeel have been applied to sliders;
* Improved UI layout for panel.
* Adjusted positioning of elements during resizing.
* New icon for "Mute" button;