lspclient update

[Mark Nauwelaerts]

Hi All,

Some more commits have added support for code actions (server suggested fixes 
for diagnostics) and dealing with the document editing that is involved.  In 
related area, also document/rename is now handled (though server-side responses 
seem not reliably comprehensive).

Now I am in the process of moving the remaining QTreeWidget to 
QTreeView/QStandardItemModel along with some other implementation cleanups in 
that area.

Regards,
Mark.