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/llmenucommands.cpp

@@ -70,6 +70,8 @@
 #include "llworldmap.h"
 #include "llfocusmgr.h"
 
+void toggle_search_floater();
+
 void handle_track_avatar(const LLUUID& agent_id, const std::string& name)
 {	
 	LLAvatarTracker::instance().track(agent_id, name);
@@ -97,7 +99,7 @@ void handle_mini_map(void*)
 
 void handle_find(void*)
 {
-	LLFloaterDirectory::toggleFind(NULL);
+	toggle_search_floater();
 }