Implemented utilities to get rid of race conditions in plugins (now used in CAR plugin only)

* Implemented LinearSmoothedValueAtomic class based on the
  JUCE 4.0 library' LinearSmoothedValue class but with some improvements
  that were suggested by Timur Doumler during his JUCE Summit 2015
  talk.
* LinearSmoothedValueAtomic is very convenient to use with float values
  that need linearly smoothed behaviour (like volume, etc.) and which
  are shared between several threads (i.e. audio and GUI).
* This class guarantees we won't have any race conditions and it will
  remains lock-free because of using std::atomic inside it.

Add new CAR files to Xcode project

CAR: fixed bug gain slider had per-channel functionality. Other minor fixes.

* ParameterSlider: added possibility to change rotary fill colour.
* CAR gain slider: changed rotary fill colour.

* Fixed bug gain slider have some buggy per-channel functionality
  so it were possibility to set different gain values for only some
  channels.
* Now gain slider affects all the channels.
* Explicitly added gain slider to the CAREditor component.

OS X packaging improvements

CAR plugin major improvements.

* Added custom audio processor editor for CAR plugin - CAREditor;
* Added buttons to switch between "Reference" and "Affected" channels;
* CAR processor some minor changes;
* Some minor changes in GenericEditor class which provide more
  possibilities for plugins (e.g. plugins can implement their own
  resized() methods, etc.);
* Added UIUtilities header file in which we can include any
  UI components/utilities which will be useful for easier plugin
  creation.

* 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.

* Since "Small text" renders very bad, without any hinting, we replace
  it by "Default" font.  And also "Default" font is much more suitable
  for Material Design.
* Adjusted font sizes.

On OS X, load plugin bundles from application bundle's PlugIns directory.  Improved error handling in PluginManager.