linkmap: Remove use of setup flags in favour of checking real attrs
linkmap: Stop deriving the corner radius from the line height

Doing this makes very little sense, and it's always going to be within
a pixel or two of the constant value. Erring on the side of a lower
rounding here.
filediff: Remove misleading comment separators

These long ago stopped actually indicating what followed.
filediff: Remove Python 2.6 compat
filediff: Comment cleaning
filediff: First pass at UI-ifying Filediff actions
filediff: Refactor next_diff to be usable for all diff traversals
filediff: Refactor chunk-based actions to remove params from callback
filediff: Refactor merging actions to remove params from callback
filediff: Migrate toggle actions to UI file
filediff: Remove cycle-pane action in favour of prev/next (bgo#701665)

Cycling panes has always felt a little bit weird, and really it's very
straightforward to use a previous/next model instead.
filediff: Add change deletion pushing, reducing pane swaps (bgo#729838)

Previously we enforced the action of pushing a change to mean that
there had to be some text to push. As it's been requested a few times,
this patch makes it so that it's possible to push an empty change as
well, which has the effect of deleting the change from the target pane.

This feels slightly odd mental-model-wise, but pragmatism beats purity
and this means that you have to switch panes much less frequently.