Property minimum, maximum and default values

[jwharm]

With this PR, it will become possible to specify minimum, maximum and default values for GObject properties declared in Java classes with the @Property annotation.

For example:

@Property(minimumValue = "1", maximumValue = "12", defaultValue = "1")
public void setMonth(int month) {
    this.month = month;
}

The @Property parameters minimumValue, maximumValue and defaultValue expect String values. They are transformed by Java-GI to the proper type using Boolean.parseBoolean(), Integer.parseInt() etcetera.

Using these three parameters, Java developers can set the minimum, maximum and default values on the GParamSpec of a property that they defined in Java.

• The minimum and maximum values are not enforced by Java-GI, so on the Java side the benefits are negligible. In most cases it is advisable to implement minimum and maximum property values in Java itself.
• The default value is returned by Java-GI for properties that have only a setter method in Java.

For properties with a getter and setter method, either both or neither methods must be annotated with @Property (or else they will not be recognized by Java-GI as a pair). The annotation parameters must be specified on the getter. They can be specified on the setter too, but only the parameters on the getter are actually used.