Deprecate qplot()

[yutannihilation]

Close #3956

TODO

• Add .Deprecate() to qplot()
• Remove stat and position and throw errors
• Tweak the document of qplot()
• Remove the use of qplot() in tests

Remaining quesitons

• How should we rewrite translate_qplot_ggplot and translate_qplot_lattice? (I decided to just delete them)
• Do we need some instruction to get rid of qplot()?
• How should we communicate with the extension package authors?

[thomasp85]

Team - should we pull the trigger on this? I don't think any more coercion without deprecation will move remaining users so there is no point in waiting longer. It is either deprecation of qplot() or leaving it in in perpetuity...

[clauswilke]

What exactly does deprecate mean at this point? Does it mean throw a warning when somebody uses qplot() or does it mean remove qplot() entirely? I think the right strategy would be to start warning people when they are using qplot() that it's on the way out. Then, in a future release, remove outright.

[yutannihilation]

throw a warning when somebody uses qplot()

This pull request was intended for this.

[clauswilke]

Then I'm in support.

[thomasp85]

Yeah, this will have to go through the proper and very very long deprecation process... but it will allow us to better ignore the function during development

[yutannihilation]

This should be ready for review now.

[thomasp85]

This looks good - had no idea we were using plot so much in the tests 😬

[yutannihilation]

Yeah, I too didn't expect I would have to modify so many lines... Thanks for reviewing, I'm merging this.