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

SignalStrengthSampler/proguard.cfg

@@ -0,0 +1,34 @@
+-optimizationpasses 5
+-dontusemixedcaseclassnames
+-dontskipnonpubliclibraryclasses
+-dontpreverify
+-verbose
+-optimizations !code/simplification/arithmetic,!field/*,!class/merging/*
+
+-keep public class * extends android.app.Activity
+-keep public class * extends android.app.Application
+-keep public class * extends android.app.Service
+-keep public class * extends android.content.BroadcastReceiver
+-keep public class * extends android.content.ContentProvider
+-keep public class com.android.vending.licensing.ILicensingService
+
+-keepclasseswithmembernames class * {
+    native <methods>;
+}
+
+-keepclasseswithmembernames class * {
+    public <init>(android.content.Context, android.util.AttributeSet);
+}
+
+-keepclasseswithmembernames class * {
+    public <init>(android.content.Context, android.util.AttributeSet, int);
+}
+
+-keepclassmembers enum * {
+    public static **[] values();
+    public static ** valueOf(java.lang.String);
+}
+
+-keep class * implements android.os.Parcelable {
+  public static final android.os.Parcelable$Creator *;
+}