DOC: Prepare What's new page for 3.4.0.

[QuLogic]

PR Summary

Still need to check whether any PRs were missed.

PR Checklist

• [n/a] Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (run flake8 on changed files to check).
• New features are documented, with examples if plot related.
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• Conforms to Matplotlib style conventions (install flake8-docstrings and run flake8 --docstring-convention=all).
• New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• [n/a] API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).

[QuLogic]

Added entries for:

• TickedStroke, a stroke style with ticks useful for depicting constraints #15458
• Add fullscreen toggle support to WxAgg backend #17843
• Use indexed color for PNG images in PDF files when possible #17895
• Support data reference for hexbin() parameter C #18103
• Created a parameter fontset that can be used in each Text element #18145
• allow webAgg to report middle click events #18172
• Replace ttconv by plain python for pdf subsetting #18181
• Support horizontalalignment in TextArea/AnchoredText. #18367
• Move PostScript Type3 subsetting to pure python. #18370
• plot directive: caption-option #18426
• Support doubleclick in webagg/nbagg #18444
• Include kerning when outputting pdf strings. #18517
• Support binary comms in nbagg. #18583
• Allow Type3 subsetting of otf fonts in pdf backend. #18651
• ENH: Add func norm #18653
• Single-line string notation for subplot_mosaic #18763
• Support fully-fractional HiDPI added in Qt 5.14. #18876
• Support raw/rgba frame format in FFMpegFileWriter #19056
• Add kerning to single-byte strings in PDFs #19582

[jklymak]

Looks great @QuLogic.