First pass at accessibility improvements for #650
Use H1 for tiddler titles

Part of #650
Use H1 for page title and H2 for tiddler titles

Ideally, we’d also rebase the headings within tiddlers so that `!
Heading` would be an H3. Hopefully we’ll do that later.
Improve a11y of close buttons in open tab

a11y === accessibility
Added aria-label attribute to link widget

And document it for the button widget
Move button language to $:/language/Buttons
Add label for close button in open sidebar tab
Add label for advanced search link
Update button widget docs
Add label and title for show/hide sidebar buttons

More fixes for #650
Merge branch 'accessibility'