We call 'update_tab_titles' a lot of times which calls 'setTabText' on
every tab. 'setTabText' calls tabSizeHint and minTabSizeHint on every
tab as well, meaning this is an n^2 operation repeated many times.
First, this prevents setTabText from being called unless it's needed,
removing most of the work done.
Second, I remove tabs in reverse, to avoid recomputing the above for
every tab on shutdown (which is at least n^3)
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