moved MainWidget class into ompl_gui package

in sdp_project_demo:
- removed MainWidget class definition
- updated creation of MainWidget object

in ompl_gui/init.py:
- include import of MainWidget

in ompl_gui/MainWidget.py:
- Added MainWidget class definition.
cleaned imports

in sdp_project_demo:
- removed unused imports Signal
- removed _fromUtf8 (remained from following tutorial with text input)

in MplCanvas.py:
- removed unused QtCore
moved PlannersWidget class into ompl_gui package

in sdp_project_demo:
- removed PlannersWidget empty class
- removed QtCore include, unused

in ompl_gui __init__.py:
- include import of PlannersWidget

in ompl_gui PlannersWidget.py:
- Added empty class definition.