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/app_settings/settings_ascent.xml

@@ -620,5 +620,27 @@
 			<key>Value</key>
 			<boolean>0</boolean>
 		</map>
+		<key>UseWebSearch</key>
+		<map>
+			<key>Comment</key>
+			<string>Always use web-based search instead of legacy search.</string>
+			<key>Persist</key>
+			<integer>1</integer>
+			<key>Type</key>
+			<string>Boolean</string>
+			<key>Value</key>
+			<boolean>0</boolean>
+		</map>
+		<key>UseWebSearchSLURL</key>
+		<map>
+			<key>Comment</key>
+			<string>When clicking search SLURLs, or using an SLapp that uses these, display the web-based search (When false, some search SLURLs may display somewhat improperly).</string>
+			<key>Persist</key>
+			<integer>1</integer>
+			<key>Type</key>
+			<string>Boolean</string>
+			<key>Value</key>
+			<boolean>0</boolean>
+		</map>
     </map>
 </llsd>