add <update check="background"> to JNLP for jfxpackager and netbeans plugin

After some investigation it is found that we can improve applets startup time (for the case when applet is cached) by doing update check in the background. This can be done by adding the following to the JNLP file:

<jnlp>
   ...
   <update check="background"/>
</jnlp>

We should add this tag as default to the jfxpackager, so all FX JNLP applets.application developed by NetBeans plugin will have this by default.

We also need an extra option for the NetBeans plugin to turn that off too, if the user wants to. (under project properties, categories, application - same place where we can specify options like pack200, self-sign, draggable, etc)