panel: Add rounded corners

panel: Add rounded corners

Current mockups show the panel curving downwards at the edges to
frame the work area and look awesome. Implement those as separate
actors to not affect the struts set by the panel, and synchronize
their state with the corresponding panel buttons so they blend in
with the panel. It might be worth considering whether the corners
should be hidden with maximized windows on the current workspace,
though this might affect the illusion of them being part of the
panel. As the corners don't affect the input region, the small
overlap with windows might not be too bad after all.

https://bugzilla.gnome.org/show_bug.cgi?id=642697