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.
Updated Polish translation
Updated Greek translation
linkmap: Reduce radius on culled chunk drawing to get a flat section
linkmap: Cairo will implicitly join strokes, so clean up these
gutterrendererchunk: Move action map into class
gutterrendererchunk: Refactor some ugliness and create new consts module
settings: Add bind_settings helper for attribute-specified properties

The idea here is to have settings bindings specified next to the
GObject attributes that they're often associated with. This commit
adds the helper, and moves all of the major uses of gsettings binding
to it.
build_helpers: Fix accidental tabs
meldwindow: 'Fix' property warning
Updated Brazilian Portuguese translation
Updated German translation
Updated Czech translation
WIP: Port to C++