Update stage-installed plugins CMakeLists.txt

Changes how staged plugins are built (resources generated and copied to [build dir]/output/python/plugins).

When _staging_ all plugins, use the following _make targets_:
*  __staged_plugins__ - stage plugins (usually after repo pull/build and project make)
*  __staged_plugins_pyc__ - stage and byte-compile all Python scripts
*  __clean_staged_plugins__ - removes the plugins directory and all contents

When _developing_ on a plugin, use the following _make targets_:
*  __staged_[plugin_dir_name]__ - stage specific plugin, regenerating any changed resources
*  __clean_staged_[plugin_dir_name]__ - removes the plugin directory and its contents

CMake options WITH_STAGED_PLUGINS and WITH_PY_COMPILE are still valid and just run _staged_plugins_ and _staged_plugins_pyc_ respectively as part of a regular project build.

In the newly available workflow for running app from build directory, leaving WITH_STAGED_PLUGINS=FALSE (should be as default now?), all above targets are still generated, but not built as part of ALL target. This allows a developer to set up their IDE with custom build process steps like so:

* make
* make staged_plugins (_optional_)
* make install (_optional_)

with _make staged_plugins_ capable of being built anytime after project build, making those project builds as fast as possible, as done here with QtCreator project setup:

![make_staged_plugins](https://f.cloud.github.com/assets/1295919/122081/4cf36540-6e2c-11e2-98c8-1afa7141e75b.png)