WIP: critical bug EXT-4837 ([NUX] When filter results in null state, provide a...

WIP: critical bug EXT-4837 ([NUX] When filter results in null state, provide a message suggesting the user try global search.)

Partial implementation of help text in case of empty list when filtered for Nearby and Recent lists of People panel:
 * Extracted support implementation of several messages for empty list depend of filtering from LLGroupList to LLFlatListEx
 * change inheritence of LLGroupList and LLAvatarList from LLFlatListView to LLFlatListViewEx
 * updated panel people to init Nearby & Recent lists with appropriate messages.

--HG--
branch : product-engine