Let pslegend -D+w accept a percentage relative to map width

[PaulWessel]

Similar to psscale, allow processing of things like +w80%. This PR implements this via a new gmt function gmt_rectangle_dimension that does the parsing, hence the PR involves psscale as well as some library and include files.

@Esteban82, All tests pass as before, but I have not tested this specifically with legend. Please give it a try. +w90% will set width to 90% of map width, while +w90%/30% will fix height at 30% of the legend width (not map width).

[Esteban82]

Thanks Paul!!

I'm affraid that is not working. I try the script below and I didn't get any legend and this message:
legend [ERROR]: Must specify -D...+w<width> if more than one symbol column (N = 5)

Full script

gmt begin GMT_earth_mask png
	gmt set GMT_THEME cookbook GMT_DATA_SERVER candidate
	gmt makecpt -Clightblue,burlywood,blue,red,white -T-0.5/4.5/1 -N
	gmt grdimage @earth_mask_05m -JQ0/15c
	gmt legend -DJBC+w90%+o0/0.05c -F+p1p <<- EOF
	N 5
	S - s 0.5c lightblue 0.25p - Ocean [0]
	S - s 0.5c burlywood 0.25p - Land [1]
	S - s 0.5c blue 0.25p - Lake [2]
	S - s 0.5c red 0.25p - Island [3]
	S - s 0.5c white 0.25p - Pond [4]
	EOF
gmt end

If a remove the line N 5, them it works but it is not the legend that I want.

[PaulWessel]

Shit, in modern mode the command has no -R -J and when that check comes it has not yet set them from hidden file... I will check. If you just try a classic case it should work.

[PaulWessel]

Just a dump 1 line error. Now rows I think (your script runs for me)

[Esteban82]

Now it works.

[Esteban82]

I took the liberty of adding the label "add-changelog".