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 update depends on [PR #44](https://github.com/ros-visualization/qt_gui_core/pull/44) to qt_gui_core.  That will need to be merged first before merging this PR.