Rename use of C++ `template` keyword in plugin API function argument
and add `G_BEGIN_DECLS` and `G_END_DECLS` to public header files to
make them easier to include in C++ code. TagManager and Scintilla
headers already have these `extern "C"` blocks so they shouldn't
require any modifications.

The Autotools build system already adds in a `dummy.cxx` to hint
Automake into C++ linking to support Scintilla, which is quite
convenient for dynamically loading of C++ plugins at run-time into
the otherwise C-only program. The other build systems seem to also
use the correct linking.