getTablePrefix() is used to show the inline legend and inline switcher.
It is not yet part of a released stable interface.
Theoretically there may be multiple text slots on a page, and we don't
want multiple inline legends. There was already a fragment assembly
system, for the benefit of hook handlers, so move that up to the page
level, so that it can also deduplicate prefix fragments coming from
each slot.
Add tests.
Bug: T324759
Change-Id: I9baa5c24128c63bc318ba13e83a024843f4ab15e
Add a legend at the top of the inline diff display, showing the
meanings of the colours of the inserted and deleted highlighting.
Also add the same text as tooltips on the highlighted elements.
The legend is added as part of a new area above the diff table
that can be modified via a new TextSlotDiffRendererTablePrefix
hook, so that extensions can add other buttons etc. there as
required.
This is a follow-up to the previous attempt, which added the
legend in DifferenceEngine::showDiff() and was called from
too many places. This patch moves it to be called in
DifferenceEngine::showDiffPage().
Bug: T324759
Change-Id: I2a3c67bcfa47313dee597e602a62073e4e298cd2
Follow-up: I6de30bf79eb5ac262285951792782b870d075e00
