Add plot markers option to plotting backends

This adds the ability to use plot markers for rqt_plot and rqt_bag for any of the available backends.  Markers are off by default but can be enabled through the backend selection dialog box.  This is most useful in rqt_bag where the plotting resolution can be modified, but may also be used in rqt_plot.  The markers appear to add quite a bit of processing in PyQtGraph, so I've added a message indicating it may add significant load for rqt_plot.

This PR depends on version 0.2.25 of <code>qt_gui_core</code>.