Optional V3 Web Search and support for search SLURLs

Added "Use web-based search instead of legacy v1 search" checkbox to System->General preferences

Unified all functions to display search floater into toggle_search_floater and LLFloaterSearch::showInstance
Added UseWebSearch and UseWebSearchSLURL debug settings for flexible configurations
- SLURLs with legacy search cannot always present the same results as web
Added SearchURL debug setting, a separate setting from the one used in the all tab, slightly different and always set by response["search"] on OSGrid logins to detect if websearch is supported
Cleaned up llfloaterdirectory includes and added "Search.WebFloater" to the CommitCallbackRegistrar in case we ever want to add a button or something to open web search (couldn't find a nice place for it, sadly), also added more functions to show specific tabs and performQueryOn to search a tab with specific query on LLPanelDirBrowser's

indra/newview/llviewermenu.cpp

@@ -188,6 +188,8 @@
 
 #include "hippogridmanager.h"
 
+void toggle_search_floater();
+
 using namespace LLOldEvents;
 using namespace LLAvatarAppearanceDefines;
 void init_client_menu(LLMenuGL* menu);
@@ -5546,7 +5548,7 @@ class LLEditSearch : public view_listener_t
 {
 	bool handleEvent(LLPointer<LLEvent> event, const LLSD& userdata)
 	{
-		LLFloaterDirectory::toggleFind(NULL);
+		toggle_search_floater();
 		return true;
 	}
 };