layouts: extend API with layouts_entity_{set,get,setpush,pullget}_kv[_ref] calls

The new calls enable to perform the get/set operations on the key/value pairs of
a particular entity in a defined layout. The push/pull operations enable to call
for the automatic update of the linked entities in the layouts (i.e. parent) for
push and for the automatic update of the requested entity using the linked
entities information for pull.

Note that 2 variations are proposed for each call, differentiated by the _ref
suffix. When _ref is suffixed, then the calls only deal with references and no
copy of the data is performed, only data references (pointers) are set/get. The
_ref suffixed calls should then be considered as more efficient but can lead to
issues if the pointers are modified after the get operations occur but before
their real use. For that reason, layouts_{lock,unlock} calls would certainly
appear in a next commit as well as _nolock suffixed calls in order to better
handle those scenarios.

Note that the ...pull/push... calls are just stubbed in that commit.
Their implementations will correspond to a dedicated commit later.