Fix doc snippets paths and parsing errors

Fix doc snippets paths and parsing errors

qtqml.qdocconf and qtquick.qdocconf now refer to the correct
snippets and source directories.

Snippet paths in .qdoc and .cpp files have been updated to refer to
the new shortened path references, e.g. \snippet qml/file.cpp instead of
\snippet doc/src/snippets/qml/file.cpp.

This also deletes snippets from src/qml/doc/snippets that belonged under
src/quick/doc/snippets (and were already duplicated there anyway) and
restores some snippet files that shouldn't have been deleted.

Also fixes some inline snippets to use \code .. \endcode instead of
\qml .. \endqml as they contained javascript or partial QML
snippets that were causing parsing errors from qdoc.

There are still snippet errors arising from qmlintro.qdoc as the
qmlintro snippets directory that it refers to cannot be located.

There are also two references to a removed snippet identifier in
examples/qml/cppextensions/plugins/plugin.cpp that need to be fixed
in conjunction with the related docs in a later commit as the relevant
code has changed and the docs are now invalid.

Task-number: QTBUG-25721

Change-Id: I50c665245a74c140470c58a32546591d187dfe4b
Reviewed-by: Chris Adams <christopher.adams@nokia.com>