initial commit of a obstacle tab

added obstacle_tab subpackage in ompl_gui:
- ObstacleWidget to contain contents of tab

added shapes.Square 
- In the future other shapes would be nice

added ObstacleWidget tab to MainWindow
- needs tweaking

TODO: functionality
Square Obstacle

Worked on square obstacle. needs to be created from dropdown selection
Disabled Tab Buttons and Tab Widget

As Pull Request #24 says only the graphical parts work so I've disabled
the buttons to avoid confusion
Merge pull request #24 from mas-group/obstacle

Obstacle Tab GUI component.

Back-end not implemented