Namespace conflicts

[teunbrand]

In #6287 we identified some problems with namespace conflicts.
In total, 53 packages were affected by these.

ggpubr::theme_transparent conflicts with ggplot2::theme_transparent.
sjPlot::set_theme conflicts with ggplot2::set_theme.
greybox::is.scale conflicts with ggplot2::is.scale.

Should ggplot2 budge or should these other packages? If ggplot2 should budge, do we have alternative names for the functions?

[teunbrand]

TODO: Send issue to sjPlot, suggesting to import selectively.