In-world feature request: Announce in chat when someone has taken a snapshot.

AnnounceSnapshots debug setting.
"Announce when someone takes a snapshot" added to Vanity Main Preferences.
took_a_snapshot translatable string added to strings.xml

Also in this commit:
Turned PlayTypingSound check into LLCachedControl.
In preparation for the crippling of all old viewers, the Vanity Main tab shall henceforth only be called Main, not Main  (General)
This change has been made in the code comments and the UI itself.

indra/newview/app_settings/settings.xml

@@ -843,6 +843,20 @@ This should be as low as possible, but too low may break functionality</string>
       <key>Value</key>
       <integer>1</integer>
     </map>
+    <key>AnnounceSnapshots</key>
+    <map>
+      <key>Comment</key>
+      <string>
+Announce if someone nearby has taken a snapshot in chat
+(Won't work on people who hide/quiet snapshots)
+      </string>
+      <key>Persist</key>
+      <integer>1</integer>
+      <key>Type</key>
+      <string>Boolean</string>
+      <key>Value</key>
+      <integer>0</integer>
+    </map>
     <key>AnnounceStreamMetadata</key>
     <map>
       <key>Comment</key>