libhawaiishell: Fix debug messages with prefixes

Use d->contentsPrefixPaths instead of prefixes.
libhawaiishell: Rename Corona to Mantle

To quote Wikipedia: in shelled molluscs, the mantle is the organ that
forms the shell.  This feels a much more appropriate name.

Issue: #148
client: Fix crash when changing shell handler

When the panel is destroyed, Qt hides the window and dockPanel() gets
called because it was connected to visibleChanged() but at this point
we already deleted m_surface.

Resolve the problem by checking whether the window is visible and
m_surface is available and it's initialized.
starthawaii: Don't use catchsegv for development builds

We want to use QtCreator to catch SIGSEGV instead.
server: Avoid crash when a panel surface is destroyed

Don't reset surface listener because panel pointer is invalid at
that point.