plug-ins: Replace guchar PDB parameters with more appropriate types

Tested this for plug-in-alienmap2 and plug-in-maze via the Python console, both of which use guchar parameters. Also tested this for script-fu-circuit, which internally uses plug-in-maze (there was no need to adjust arguments there).

There is currently only one PDB procedure left using guchar, which is plug-in-exchange. If appropriate, the guchar parameters there can be changed to integers with values limited to 0-255.

The problem with guchar is that it is currently not supported as a GimpProcedureConfig property, meaning that guchar PDB parameters cannot be modified before running a PDB procedure. If it is desirable allow modifying guchar via config, I have a commit ready and can create another merge request for that.