UserLibrary workspace enhancements

[luca-domenichini]

This feature will provide some enhancement to the workspace management of custom controls.

• users can link external jars/fxmls or folders to a workspace that is saved in a xml file inside the userlibrary folder. Jars/fxmls are not copied inside the library folder but are just linked as an external source.
• ability to import/export a workspace
• command line parsing the workspace: a new switch --ws to instruct SB to load that workspace (e.g. in Windows launching SceneBuilder.exe --ws="C:\Users\donaldduck\myworkspace.xml")
• the last workspace imported in SB should be persistent, meaning that it would behave like regular jars/folders added to the library as before. The workspace can be altered by using an import or by launching SB with the command line switch --ws
• workspace is saved in xml format using XStream library (gradle "com.thoughtworks.xstream:xstream:)

This is the concept of what I am proposing (taken from #236 -- I'm the author of that too..)

Current Behavior

Currently there is no easy way to switch from a set of libraries imported into SB to another set.
That would be useful when working on different projects, e.g. because different projects requires different libraries in different versions.
A workaround to this is to have some batch that clear the jars and copies all of them under the library folder before launching SB.

[luca-domenichini]

To avoid conflicts with other PRs, I will wait to provide a PR until #387 #389 and #391 are merged

[luca-domenichini]

@abhinayagarwal would be really great to make it for SB 17 🚀
Please, review and merge #387 #389 and #391 and I'll submit this PR blazing fastly 🔥