Hold set to True, holds all plots

This fixes displaying multiple plots but now it never clears old plots. 

working on #27
GUI - Clear Plot and initial Bounds

Clear Plan Button actually cleared the Plot. Renamed.
Moved connection into MainWidget.py from sdp_project_demo.py

Bounds are now initialized as default values not zero.
Grey out plot button until plans are available

Closes #29

Function to enable/disable the plot button already existed we just 
hadn't used it or disabled the plot button on init. before solve function
finishes it checks if paths list is empty and sets the plot button.
Added button to clear plans list

Multi plots #28 requires a way to clear the list of found paths.
The button is disabled by default, like #29 suggests for plot button.

The button needs to be connected
Added Labels to plot

This Closes #30

Also removed boundsChanged PyQtSignals. towards #27
refresh should not reset the bounds, this results in MatPlotLib using
automatic axis.
Removed Connections between MPLViewer and Bounds

This Closes #27

The plot viewer should not be able to set the bounds. Only the user 
should set the bounds. This fixes the connections issues from #27. 

Added getBounds function to boundsWidget. 

The bounds need to be passed to MPLViewer every time it plots.
Otherwise MatPlotLib will try to use smart automatic axis values.

added setBounds in sdp_project_demo. bounds are set when solve
calls setupPlanners. Solve enables the plot button. The bounds are
used when the plot button is clicked.
Merge pull request #31 from mas-group/GUI-multi-plots

working gui with multiple planners