e.g. accept the first character always to start the autocompletion
but after that refer to the plainpart logic to split up in
new pill candidates. Also, don't allow merging adjacent parts.
This makes sure that "@nonmatchingfoo @someuser"
only matches "someuser" in the autocomplete.
the avatar style code is still different though,
as it's implemented differently
This also prevents updating the css variables when not needed,
which caused the avatar to flicker when updating the editor.
You can't append to it anyway, so mark it uneditable and skip
uneditable parts if that's where an edit ended up.
This has the added advantage that if there is text after a newly
insert pill, the cursor will be put just before it rather than
in the pill, after the last character.
