In order to avoid code replication, use a template-based solution for

In order to avoid code replication, use a template-based solution for
all types requiring the exact same code, as suggested by Tommaso.
This allows to simply add a single-line instantiation declaration
if there's the need of adding another type requiring the same code.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@37366 a592a061-630c-0410-9148-cb99ea01b6c8