More OSX build fixes.

* Many path related fixes.
* Fix execution of external applications.
* Update mac-osx.txt.
* Add top-level links for standalone applications inside OSX bundle.
* Fix document icons for Eeschema and pl_editor.
* Create individual bundles for standalone applications inside the main application bundle.
* Add usual 'site-packages' to python path in OSX bundle.
* Fix name of OSX bundle plugin folder.