Centralize type conversion functions.

This adds the common GURL->QUrl and string16->QString conversions
into a common header and use those functions throughout the code.
Move the qStringToStringType into the same header and rename it
to a name consistent with the others.

This also cleans up shared_globals.h by moving content:: forward
declarations in the cpp, where they are used.

Change-Id: I19527ea7de1f6047aae8b44c97eb4d7c3e5a0e54
Reviewed-by: Andras Becsi <andras.becsi@digia.com>