Add "hide fields" filter to outline view's toolbar

[travkin79]

I'd like to add a toolbar action to the outline view for hiding fields. This filter is very common and is available, e.g. for Java (JDT) and C++ (CDT). It would look like very much like in JDT (selected button in the following screenshot):

Are there any objections on implementing that filter in LSP4E (for all language servers)?

This is a follow-up issue to #254.

[rubenporras]

That would be appreciated from my side. Which elements are you planning to offer hiding with an icon?

[travkin79]

For now, I would only add one button "hide fields".