Fixes a bug with my previous change that would crash the program if you
tried to change the parameters of a processor without a parameterEditor.

Allows parameters to be set from saved XML and changes the
parameterEditor buttons (currently for discrete parameters only) to
reflect which parameters are currently used by selected and
non-selected channels. This fixes a problem where the buttons would
always show what the last press was rather than the current values for
the active channels (e.g. If you changed the parameters on Channel 2,
and then switched to select only Channel 1, the GUI would incorrectly
indicate that the settings for Channel 2 were active). Additionally,
allows the user to see what parameter values are used for channels not
currently selected.

For some reason, "getCustomTypefaceForFont" wasn't being called on OS X.
It probably has something to do with platform-specific code for loading
fonts. Now, in cases where custom fonts are needed, loading requires
4 lines instead of 1. Kind of a pain, so hopefully this is a temporary
solution until we can figure out what's different about Macs.

It's unused, declared obsolete, and is triggering GCC warnings. If
anyone wants it back, it'll be in the Git history.

Signed-off-by: Marti Bolivar <mbolivar@leaflabs.com>