Processing toolbox: support disabling some functions based on layer data

[mfisher87]

I think we can get there by making layer processing functions more declarative in a data structure. For example, a mapping like:

PROCESSING_TOOLBOX = {
  'buffer': {
    title: '...'
    description: '...',
    predicate: isVectorLayer,
    gdalCommandTemplate: [ ... ],
  },
  'extractCentroids': {
    title: 'Extract centroids',
    description: 'Calculate centroid points for each feature in the layer',
    predicate: isPolygonLayer,
    gdalCommandTemplate: [ ... ],
  },
 // ...
}

A predicate function could be any function that accepts a layer and returns a boolean.

See post by @arjxn-py in #744 (review)