Settings: added default sharing track file format.

Fixes #225.

src/main/res/values/settings.xml

@@ -220,5 +220,7 @@
         <item>@string/settings_recording_track_name_number_option</item>
     </string-array>
 
+    <string name="export_trackfileformat_key" translatable="false">exportTrackFileFormat</string>
+
     <string name="settings_recording_key" translatable="false">settingsRecordingKey</string>
 </resources>

src/main/res/values/strings.xml

@@ -333,6 +333,7 @@ limitations under the License.
     <string name="settings_show">Show</string>
     <!-- Settings Advanced -->
     <string name="settings_advanced">Advanced</string>
+    <string name="settings_default_trackfileformat">Export/sharing file format</string>
     <!-- Settings Chart -->
     <string name="settings_chart_by_distance">By distance</string>
     <string name="settings_chart_by_time">By time</string>

src/main/res/xml/settings.xml

@@ -143,6 +143,10 @@ limitations under the License.
                 android:targetClass="de.dennisguse.opentracks.io.file.exporter.ExportActivity"
                 android:targetPackage="@string/applicationId" />
         </Preference>
+        <ListPreference
+            android:key="@string/export_trackfileformat_key"
+            android:title="@string/settings_default_trackfileformat"
+            app:useSimpleSummaryProvider="true" />
     </PreferenceCategory>
 
     <PreferenceCategory android:title="@string/settings_reset">