moved GUI Widget classes used for matplotlib viewer into subpackage

moved:
- MplCanvas -> ./plot_view/MplCanvas
- MPLViewer -> ./plot_view/MPLViewer
- MyStaticMplCanvas -> ./plot_view/MyStaticMplCanvas

created ./plot_view/__init__:
- imports MPLViewer

in ompl_gui/__init__:
- updated import of MPLViewer
- commented out unused imports

in MainWidget:
- updated import of MPLViewer
Moved Problem Tab related classes into subpackage problem_tab

Created new subpackage:
- moved ProblemWidget into problem_tab
- created problem_tab/__init__: which imports ProblemWidget

in MainWidget:
- updated ProblemWidget import

in ompl_gui/__init__
- updated ProblemWidget import
Moved Bounds Tab related classes into subpackage bounds_tab

moved:
- BoundsWidget -> ./bounds_tab/BoundsWidget
- BoundsBox -> ./bounds_tab/BoundsBox

create bounds_tab/__init__:
- import BoundsWidget

updated:
- __init__ -> import BoundsWidget from bounds_tab
- MainWidget -> import BoundsWidget from bounds_tab
- MainWidget -> BoundsBox import not required
Moved PlannersWidget tab classes into subpackage

Moved:
- PlannersWidget -> ./planner_tab/PlannerWidget
- PlannerBox -> ./planner_tab/PlannerBox
- OptionBox -> ./OptionBox
- ControlBox -> ./ControlBox

Created planner_tab/__init__:
- added PlannerWidget import

In ompl_gui/__init__:
- updated PlannerWidget import
- updated __ALL__ -> removed moved classes

In MainWidget:
- updated PlannerWidget import
Merge pull request #16 from mas-group/pkg-restruct

Restructure ompl_gui package into subpackages base on each tab