Bug 1687263: part 4) Defer and in some cases avoid removing spellchecking-ranges...

Bug 1687263: part 4) Defer and in some cases avoid removing spellchecking-ranges. r=smaug

Removing ranges is expensive. This reduces flickering of the spelling
mistakes and prevents 100% CPU usage of 1 core on slower machines.

The essence of this patch is, that when text doesn't change, all
existing spellchecking ranges are reused.

Before this patch, removing ranges was done as part of the time slice in
`mozInlineSpellChecker::SpellCheckSlice`. That is, slow removals of
ranges contributed to the amount of words to be spellchecked
asynchronously.
Therefore, the amount of words to be spellchecked in one chunk could
increase from the minimum, `INLINESPELL_MINIMUM_WORDS_BEFORE_TIMEOUT` to
the maxium `INLINESPELL_MAXIMUM_CHUNKED_WORDS_PER_TASK`.
Consequently, the asynchronous checking might take longer. If that turns
out to be problematic, reducing
`INLINESPELL_MAXIMUM_CHUNKED_WORDS_PER_TASK` could be a fix.

Differential Revision: https://phabricator.services.mozilla.com/D116096