Take the word under the cursor from the shared word rule.

[ilya-fedin]

The word under the cursor was taken with
QTextCursor::select(QTextCursor::WordUnderCursor) in two places - when a range
is re-checked after an edit, and when the context menu is filled. That path goes
through QTextEngine::atWordSeparator(), a hardcoded list of ASCII punctuation
that has the apostrophe in it, so don't was cut down to don: the suggestions
were offered for a piece of the word, and "add to dictionary" added that piece.
One of the Qt patches of the official builds is what makes it behave; on an
unpatched Qt the menu and the underline disagree - donn't shows it.

WordAtPosition() now answers it with Ui::Text::IsWordSeparator(), which knows
the same rule the patch teaches Qt: an apostrophe is a separator only when it is
doubled or when it stands at the edge of a word. The whole module then agrees
about what a word is, without the patch.

RangesFromText() is left alone: splitting a whole text needs the Unicode word
boundaries, which a list of separators cannot replace for the scripts that do
not put spaces between words.

The text is read a character at a time, through QTextDocument::characterAt(),
so a block is not copied for every question. Measured on the same machine with
the flags of the release build (-O3 -flto, static Qt), per call:

block	WordUnderCursor	this
128	558 ns	398 ns
512	548 ns	396 ns
4096	536 ns	402 ns
8192	534 ns	404 ns
16384	563 ns	400 ns

It also keeps the convention of WordUnderCursor - when the position is between
two words, the one on the left is answered - because the caller compensates for
it (isPosNotInWord in contentsChange()). Verified against it on every
position of a set of samples.

Needs desktop-app/lib_ui#352.