add a 'clear logs' button to the log window

services/sync/locales/en-US/log.dtd

@@ -1,4 +1,5 @@
 <!ENTITY dialog.title         "Mozilla Weave Sync - Activity Log">
 <!ENTITY saveAsButton.label   "Save As...">
+<!ENTITY clearButton.label    "Clear Logs">
 <!ENTITY briefLogTab.label    "Brief Log">
 <!ENTITY verboseLogTab.label  "Verbose Log">