Allow adding custom constraints

[kamilburda]

The GIMP PDB contains many procedures returning value(s), such as obtaining layer dimensions, whether the layer has an alpha channel, etc. This information could potentially be used as custom constraints, e.g. "layer width is less than 500 pixels". Generally speaking, the custom constraints should support operators applied on the procedure return values (equals, less than, greater than, etc.).

For greater flexibility, placeholders should be supported as described in #2.

The problem with supporting custom constraints is the fact that some plug-ins which modify layers destructively may also return values, meaning such constraints act like procedures when they are not meant to.

Additionally, it could be useful sometimes to combine multiple constraints (built-in or custom) with "OR" or "AND" operators of sorts, to create constraints like "layer width is less than 500 pixels OR layer height is less than 500 pixels".