Issue 45: Add ImportAllTracks to import GPX files from /sdcard/MyTracks/gpx

ImportAllTracks is essentially just a modification of ExportAllTracks that
uses the existing GpxImporter to import all GPX files found in
/sdcard/MyTracks/gpx. So that the import functionality can always be accessed,
the MyTracks Activity's MenuManager is altered so that the Tracks button shows
regardless of whether there are any tracks in the database (ImportAllTracks is
launched by a button in the lower right of the MyTracksList activity).

A Toast message shows if no GPX files are found; this serves as a hint to
users on where to put GPX files for import so that this addition should be
discoverable and useable without documentation.

Im

MyTracksLib/res/values/strings.xml

@@ -0,0 +1,19 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+Copyright 2010 Google Inc.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+     http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+-->
+<resources>
+  <string name="lib_name">My Tracks Base Library</string>
+</resources>