kate: register as plugin

kate: register as plugin

KTextEditor::Factory does its own thing in terms of plugin lookup, view
creation and such. the plan is to remove the KTextEditor interface glue
code (because there is only one part providing the functionality for that
interface - katepart) and register KateDocument as part plugin via
K_PLUGIN_FACTORY() macro but until then that will work

fixes loading of the part in some cases (e.g. when Ark attempts to load it
from its previewer)

Signed-off-by: Ivailo Monev <xakepa10@gmail.com>