Plugins which in turn want to open other interfaces need some way to
access the interfaces system itself. This is realized with a 'interface'
implementing the interface api and passed to the plugins initialization
routine.
A plugin which wishes to open other interfaces must
safe this in a local variable defined by LUMIERA_PLUGIN_INTERFACEHANDLE
with the LUMIERA_PLUGIN_STORE_INTERFACEHANDLE macro.
The LUMIERA_INTERFACE_OPEN and LUMIERA_INTERFACE_CLOSE macros change their
definition depending on compiled as buildin or plugin to account for that.
This starts the proposed changes in
http://www.pipapo.org/pipawiki/Lumiera/DesignProcess/ApplicationStructure
* Moving all necessary files from src/backend to src/lumiera
* create a small helloworld main app
* setup the buildsystem to build it all
* fix include paths at many places
The testsuite is not yet fixed here, that will be the next commit
