[IMP] runtime: allow validating object values using a type description

In JS, objects are often used as a mapping that gives fast lookup on the
keys, in those cases, the keys themselves are not known in advance, but
the values may have a shape that they're expected to follow. This commit
adds support for a `values` key in schema for objects, which will be
checked against all values in the object during validation.

doc/reference/props.md

@@ -212,6 +212,7 @@ For each key, a `prop` definition is either a boolean, a constructor, a list of
   - `type`: the main type of the prop being validated
   - `element`: if the type was `Array`, then the `element` key describes the type of each element in the array. If it is not set, then we only validate the array, not its elements,
   - `shape`: if the type was `Object`, then the `shape` key describes the interface of the object. If it is not set, then we only validate the object, not its elements,
+  - `values`: if the type was `Object`, then the `values` key describes the interface of values in the object, this allows validating objects that are used as mappings, where keys are not known in advance but the shape of the values is.
   - `validate`: this is a function which should return a boolean to determine if
     the value is valid or not. Useful for custom validation logic.
   - `optional`: if true, the prop is not mandatory
@@ -276,6 +277,10 @@ class ComponentB extends owl.Component {
         name: {type: String, optional: true},
         url: String
       ]},    // object, with keys id (number), name (string, optional) and url (string)
+    someObj3: {
+      type: Object,
+      values: { type: Array, element: String },
+    }, // object with arbitary keys where values are arrays of strings
     someFlag: Boolean,     // a boolean, mandatory (even if `false`)
     someVal: [Boolean, Date],   // either a boolean or a date
     otherValue: true,     // indicates that it is a prop