check for complete pyside presence before trying to import

importing pyside partially and then falling back to pyqt causes a crash
in sip (see gh-1431)
To avoid it try to locate all modules before the import and should that
fail print a warning.
Merge pull request #2757 from juliantaylor/incomplete-pyside

check for complete pyside presence before trying to import
Merge pull request #2396 from Carreau/json_schema

create a ipynbv3 json schema and a validator
Merge pull request #2333 from Carreau/metaui

UI For Metadata
add slideshow extension/preset for celltoolbar