build: Update build files for cleaner build output
src: Use proper boolean when relevant
core: Add small double-linked list helpers
core: Maintain an internal list of contexts
test: Fix small naming detail in glib_test
build: Name plugins according to their type
src: Check DBus connection to know about library initialization
core: Move backend method variable

This prepares for runtime backend method switch.
core: Add a new event when no backend is currently there

This prepares for runtime backend method switch
core: Make backend unusable when relevant
backend: Load all backend and implement transparent hand-over

Now if a backend goes away/comes back, this will not be an error.
In addition of that, all available backend plugins are loaded, so it is
possible to switch from a backend to another.
api: Only one event callback, but for property, is really necessary

This replaces error/connected and disconnected callbacks by a unique
event callback.
core: Add an internal debug messagse mechanism

A DBG() can be used to output debug message, when --enable-debug is in
use.
todo: Remove the debug output task
src: Copyright update
test: Remove obsolete python examples
header: Remove extra space
test: Manage CONNLINE_EVENT_NO_BACKEND in test files
dbus: Fix connline_dbus_get_array so it sets length properly
include: Fix small issues in headers