vcview: Hand off some labelling metadata from VC to file comparisons
data/ui/filediff.ui: Add toolbar items for label widgets
Initial pass at actually setting labels
data/ui/filediff.ui: Pad set labels for appearance purposes
filediff: Fix interaction between set labels and save-as action

When setting fixed labels, we don't expect the user to change the
file, and there's no UI for doing so. However, it's still possible
(and reasonable) to change the file path by saving a labelled pane as
a different path. In this case, we already correctly set the path on
the file selector, so we just need to re-show it, in case the pane is
actually labelled.
vcview: Support setting tab label, improve version labels (bgo#699056)

Having good labels that work for all VCs is hard, so these strings are
a compromise. Setting the tab label automatically based on the strings
used for the pane labels isn't sensible, so we now specify that as
well.

This commit also fixes some very confused ordering of local/remote
labels for versions.