Add button to reset parameter default values

[imagejan]

Currently, the parameter input harvested from @ parameters is remembered between script executions and between separate sessions. While that is useful for repeated executions of the same script, there should be a way to restore the defaults defined by value = someValue in the parameter definition, e.g. by adding a Reset button.

[ctrueden]

The relevant code for a "reset" option to call would be PrefService#clear(Class), where Class is the Java class of the Command implementation to clear.

However, for scripts, that ends up always being ScriptModule. This is actually a big bug! We shouldn't be using the same preferences area for every single script—as things stand, any parameter called e.g. a—regardless of type—will have its value persisted in the same place. I filed scijava/scijava-common#193 to track this.