Optional loading of JQuery and JQuery-ui

[rlsutton1]

GridStack when loaded automatically loads it's versions of JQuery and JQuery-UI.

This would be fine except that my project already uses JQuery outside of the scope of Vaadin, and when GridStack loads JQuery it is a different version (not such a big problem) and removes my plugins which I've added to JQuery which leaves me referencing non-existant plugins (JQuery.myPlugin.myMethod())

Also the my project uses a customized JQuery-UI, when GridStack replaces JQuery-UI the various extensions to JQuery-UI I rely on get removed.

I have found that GridStack will operate with the versions of JQuery and JQuery-UI I already have in my project.

I will create a pull-request with a fix for this, allowing control over the loading of JQuery and JQuery-UI

[rlsutton1]

I have created a pull request for this issue