Population Workflow / Time profile: Observed data legend caption

[Yuri05]

In the plots with the reference population: it's not clear if the observed data belongs to the reference population or not.

We could e.g. add the population name in brackets for the observed data in the same manner as for the simulated data.

[pchelle]

@Yuri05
The CRAN package {ggtext} has functions that can be used to render captions, labels and watermark from markdown/html text.

If interested, we could use it to render reference populations in bold, italic or/and in different colors.
The change would also allow simple ways to include _subscript and ^superscript in the titles, captions, etc.
We could also use the feature to include images in watermark

[pchelle]

Should be fixed by PR #939