CMake: Set an output directory for plugins.

CMake: Set an output directory for plugins.

Redefine PLUGIN_DIR similar to DATAFILE_DIR and use it on all
platforms. Add WiresharkPlugin.cmake so that we can start defining common
macros for plugins/*/CMakeLists.txt. Load plugins in out-of-tree builds.

Change-Id: I8c1359ed3cf8a71788b8320ff89dfe2d3969def2
Reviewed-on: https://code.wireshark.org/review/6640
Reviewed-by: Gerald Combs <gerald@wireshark.org>