Make the rich message editor accessible to screen readers - #2168

A rich message is written as a column of blocks: paragraphs, headings,
quotes, code, lists and checklists, toggles, tables, formulas, dividers,
photos and videos, music, files and maps. Much of it was closed to a
screen reader.

The blocks are put in order by holding one and dragging it, and thrown
away by dragging it onto a bin, by a finger alone. A list in order by
dragging was given moves for a screen reader elsewhere, but here they
would have skipped what the editor does after a drop, such as taking a
block into a quote it was dropped in.

Every text read the same, so a heading, a quote, a line of code and a
list item could not be told apart. A checklist box was a button with no
name and no state, and the arrow that opens and closes a toggle was a
button with no name.

A divider, a formula, a file, a piece of music, and the photos and
videos of a block are drawn by hand, so they were nothing at all: they
could not be found, let alone moved or thrown away. The button that
plays the music, the one that opens or downloads a file, the formula
that opens its editor when pressed, and a photo that opens the picker
when it is empty or stops its upload while it goes up, all answered a
finger alone. The pages of a slideshow turn by a swipe alone, and the
button that turns a group of photos into a slideshow and back was
called More options.

In a table, the handles along the edges that open the options of a row
or a column are drawn and pressed by a finger alone, so rows and
columns could not be added, removed or merged, and a cell did not say
where in the table it stood.

Now each block carries actions that move it up or down and, for the
ones that are not text, throw it away, through the very calls a drag
and a drop on the bin make, with the editor's own care after them.
A text says the kind of block it is in, a checklist box is a checkbox
that says if it is checked, and the toggle arrow says what it does and
whether it is open. The blocks drawn by hand are read with what they
are and what they hold, and press as a finger does. The music can be
played, paused and moved through. The photos of a block are read one
by one with their state, and a slideshow turns by the scroll gestures.
The slideshow button says what it will do. A table cell says its row
and column, and has actions that open the options of its row, its
column or itself, as the handles do. The six alignment buttons of that
menu, icons alone, say the way each one aligns.