minimize dock-widgets in a statusbar

User has option of minimizing dock widgets. Also he can
enter fullscreen mode by going to the View menu or by
pressing F11. When exiting fullscreen mode the state will
be the one before the fullscreen. If some widgets were
hidden before they will remain that way after.

Signed-off-by: Marius Avram <marius.avram1309@gmail.com>
fixed issue 259
added mechanism to find the appleseed::studio::RenderTab associated to a given tab.
some minor fixes
Merge pull request #392 from sriravic/upstream

Added a toolbar button to clear render widgets to black (#259).
Fix styling issues and tiny bug

- fix styling issues for the minimizing widgets code
- force fullscreen when some widgets have been manually brought
on screen (opposite of manual minimization).
Merge pull request #391 from marius-avram/B331-NEW

minimize dock-widgets in a statusbar
fixed compilation on Windows.