plot: test glyphboxes for plain and latex handlers

[sbinet]

working on #708 uncovered a slew of bugs related to plot.GlyphBox{,er}.
we should probably test more thoroughly the various modes in which plot.GlyphBoxes are involved:

• with a text.Plain handler (the default)
• with a text.Latex handler
• with and without a plot.Plot.Title
• with and without a plot.Plot.{X,Y}.Label
• with and without rotated labels, title and legend entries (plot: wrong glyphboxes for rotated axes labels or title #722, plot: glyphbox for Legend #724)
• with and without a centered plot.Plot.{X,Y}.Label (+ rotated or not?) (plot: wrong glyphbox for non-centered axes labels #723)