Accessibility fixes batch 1

[mlewand]

This is an epic to list of accessibility issues that we'd like to solve first.

The parent ticket covering the entire topic: #11712.

ToC

• Focus in dropdowns:
  • The placement of focus after opening dropdowns #11838 
  • What should happen with the focus after someone opened a dropdown (with the mouse)? #2000
  • Dropdowns should close on Esc press #11568 
  • Typing when a dropdown is open #3215
  • The selected language in code block dropdown is not visible #5859 
  • Improve accessibility of drop–downs #1037
• Grids:
  • Accessibility of the tables UI #3184 (DUP of the other ones)
  • Discovery: research keyboard handling for grid components #11851 
  • Add keyboard support for insert table dropdown #3176 
  • Cannot change list style with keyboard #11041
• Focusing toolbars:
  • It's impossible to focus the toolbar with the keyboard in the source editing mode #10368 
  • Unable to focus balloon toolbars (image toolbar, table toolbar etc) #5146
• Colors:
  • Research options to improve contrast for our toggle buttons #11989
  • The state of toggle buttons is conveyed only via color with low contrast #1405
• Misc:
  • Toolbar button tooltips should also be visible for keyboard users #5581 
  • Toolbar button tooltips should also be visible for keyboard users – let's try again #12067 
  • Buttons with dropdown should feature proper semantic markup #12258

[Reinmar]

All the issues from this Epic except #2045 which is blocked by #11438 were resolved so I'm closing it.