[FR] PGFPlotsX backend: respect valign argument for annotations

[avonmoll]

When adding an annotation to a plot, e.g.,

annotate!(0, 0, ("Interesting point", :right, :top))

only the halign=:right argument for font ends up getting passed thru to PGFPlotsX. This FR is asking for the :valign=:top argument to be passed to the backend as well (as it does, e.g., when using GR).