Plot Appearance Improvement: Eliminate border and spacing around plot and axes.
Bug Number: n/a
Release notes (y/n): Yes
For Release Nums: 6.8

The border and spacing around and inside plot panels, and in between the 
plot panel and the axes, have been removed.  This cleans up the appearance 
of RiverWare plots (currently implemented with the Qwt 6.1.2 library). 
This is a followup from updating our Qwt library from Qwt 5.2.3 to Qwt 
6.1.2, also being done for RiverWare 6.8.

This GIT commit is a squash merge from the "PlotEdgeTest" branch.

These spacing and border changes are internally presented using terms 
defined in the "CSS Box Model."  (Google that, or see the following image 
link).  These terms are "margin, border, and padding".  
http://cadswes2.colorado.edu/~philw/2015/Qwt/GenImages/CssBoxModel.gif

These graphical provisions can experimentally be re-enabled with a 
RiverWare build having the PlotDialog's "Test" menu shown.  This menu 
contains three checkboxes: "Show Plot Margin," "Show Plot Border," and 
"Show Plot Padding".  To show the plot dialog's test menu, change the 
following symbol in Q3GUI/PlotDialog.cpp:

   static const bool SHOW_TEST_MENU (false);   // release: false

The eliminating of the margin, border, and padding in plots is effected 
with the following initializations in Q3GUI/SlotPlot.cpp (and NOT showing 
the PlotDialog's "Test" menu, so that the current values cannot be changed):

   bool SlotPlot::_showPlotMarginDefault (false);   // release: false
   bool SlotPlot::_showPlotBorderDefault (false);   // release: false
   bool SlotPlot::_showPlotPaddingDefault (false);  // release: false

There was a question about whether it was appropriate to wipe out the 
plot's Padding (inside spacing, between the Border and content within the 
plot panel).  With the Padding set to zero, series curves can, for example, 
run right along the edge of the plot after operating "Auto-Scaling". We 
are deciding that this is actually desirable.  See #2 of the four options 
presented on this set of webpages:

   http://cadswes2.colorado.edu/~philw/2015/Plotting/TightenMargins/Test2.html

See also screenshots of our initial demonstration of independently removing 
the Margin, Border and Padding:

    http://cadswes2.colorado.edu/~philw/2015/PlotEdgeDemo/edge.html

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