add option to provide an option for color fill in some graphs

[dericed]

for instance in the "Y" plots, it derives from:

qctools/Source/Core/VideoCore.cpp

Lines 274 to 278 in 0573c33

	{ Group_Y, Group_VideoMax, "Y MIN", "lavfi.signalstats.YMIN", 0, false, DBL_MAX, DBL_MAX, ActiveFilter_Video_signalstats },
	{ Group_Y, Group_VideoMax, "Y LOW", "lavfi.signalstats.YLOW", 0, false, DBL_MAX, DBL_MAX, ActiveFilter_Video_signalstats },
	{ Group_Y, Group_VideoMax, "Y AVG", "lavfi.signalstats.YAVG", 0, false, DBL_MAX, DBL_MAX, ActiveFilter_Video_signalstats },
	{ Group_Y, Group_VideoMax, "Y HIGH", "lavfi.signalstats.YHIGH", 0, false, DBL_MAX, DBL_MAX, ActiveFilter_Video_signalstats },
	{ Group_Y, Group_VideoMax, "Y MAX", "lavfi.signalstats.YMAX", 0, true, DBL_MAX, DBL_MAX, ActiveFilter_Video_signalstats },

I'd like to specify colors (including alpha) to fill between each plot and the plot or graph border above or below.

For instance:

plot    color fill below
Y-MIN   clear
Y-LOW   [email protected]
Y-AVG   [email protected]
Y-HIGH  [email protected]
Y-MAX   clear,[email protected]

and
fill above top graph = clear

[ElderOrb]

hmm, not sure such a functionality supported by Qwt, but will check

[ElderOrb]

[dericed]

done in #826