Fix javadoc - the name of the JavaScript function to submit a form is just 'submit'

(cherry picked from commit 35c80feb09986c36239257975ca9824f1041cb67)
Upgrade Jetty to 9.2.0
Update ResourceStreamLocator's javadoc to use the resource reference variation too

(cherry picked from commit 84d49a08ea8febec6935ab49e8d1ff2986a9af4d)
WICKET-5240 Add ModificationWatcher implementation that uses NIO2 file system polling

Allow the poll frequence to be configurable.
Add support for modification and deletion events