This allows us to provide a bit more context to things in the descendants
callgraph based on the symbol. Context switches get specialized highlight
as well as All Processes and Processes nodes.
This is ultimately to replace libsysprof-ui, and will focus on the display
of capture files. Recording will be left to the sysprof application in
terms of UI, and other applications like IDEs will use the
libsysprof-profile API directly along with libsysprof-gtk for visualizing.
