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
1eb586013c