Display "Migration in progress — do not interrupt…" during migration

Would be nice to have a determinate progress meter on Windows, but that
will require enumerating all files, which maybe would just make things
slower. Might be safer, though, by discouraging people from
force-quitting.

chrome/content/zotero/xpcom/zotero.js

@@ -1466,9 +1466,15 @@ Components.utils.import("resource://gre/modules/osfile.jsm");
 			partial = true;
 		}
 
+		// Not yet used
+		let progressHandler = function (progress, progressMax) {
+			this.updateZoteroPaneProgressMeter(Math.round(progress / progressMax));
+		}.bind(this);
+
 		let errors;
 		try {
-			errors = yield Zotero.migrateDataDirectory(oldDir, newDir, partial);
+			this.showZoteroPaneProgressMeter(Zotero.getString("dataDir.migration.inProgress"));
+			errors = yield Zotero.migrateDataDirectory(oldDir, newDir, partial, progressHandler);
 		}
 		catch (e) {
 			// Complete failure (failed to create new directory, copy marker, or move database)

chrome/locale/en-US/zotero/zotero.properties

@@ -131,6 +131,7 @@ dataDir.selectedDirEmpty.useNewDir             = Use the new directory?
 dataDir.moveFilesToNewLocation                 = Be sure to move files from your existing Zotero data directory to the new location before reopening %1$S.
 dataDir.incompatibleDbVersion.title		= Incompatible Database Version
 dataDir.incompatibleDbVersion.text		= The currently selected data directory is not compatible with Zotero Standalone, which can share a database only with Zotero for Firefox 2.1b3 or later.\n\nUpgrade to the latest version of Zotero for Firefox first or select a different data directory for use with Zotero Standalone.
+dataDir.migration.inProgress                               = Migration in progress — do not interrupt…
 dataDir.migration.failure.title                            = Data Directory Migration Error
 dataDir.migration.failure.partial.text                     = Some files in your old %1$S data directory could not be transferred to the new location. You should close %2$S and attempt to move the remaining files manually.
 dataDir.migration.failure.partial.old                      = Old directory: %S