ENH: Clean-up code in LineGrabPlot

[cbertinato]

• Evaluate whether picker interface is more efficient
• Work on modularity
• Work on speed

[bradyzp]

I'm going to start some work related to this feature, as I think it will be useful for the Transform tab interface.

Specifically to re-implement the graphical decimation routines to improve speed when interacting with the charts.

Also to improve the modularity of the code as it comes to adding/removing sub-plots; with some possible work towards adding the Twin-Y axes scales.

Goals

• Enable graphical decimation
• Enable dynamic addition/removal of sub-plots
• Implement Twin-Y axis on plots/sub-plots.

[bradyzp]

Resolved in PR #53
Future improvements to be tracked in separate issues.