Theme.MaterialDialog needs to be AppCompat (#470)

When trying to create widget:
10-28 15:12:24.529 26747 26747 E AndroidRuntime: FATAL EXCEPTION: main
10-28 15:12:24.529 26747 26747 E AndroidRuntime: Process: org.dmfs.tasks, PID: 26747
10-28 15:12:24.529 26747 26747 E AndroidRuntime: java.lang.RuntimeException: Unable to start activity ComponentInfo{org.dmfs.tasks/org.dmfs.tasks.homescreen.TaskListWidgetSettingsActivity}: java.lang.IllegalStateException: You need to use a Theme.AppCompat theme (or descendant) with this activity.
10-28 15:12:24.529 26747 26747 E AndroidRuntime: 	at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2817)
10-28 15:12:24.529 26747 26747 E AndroidRuntime: 	at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2892)
10-28 15:12:24.529 26747 26747 E AndroidRuntime: 	at android.app.ActivityThread.-wrap11(Unknown Source:0)
10-28 15:12:24.529 26747 26747 E AndroidRuntime: 	at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1593)
10-28 15:12:24.529 26747 26747 E AndroidRuntime: 	at android.os.Handler.dispatchMessage(Handler.java:105)
10-28 15:12:24.529 26747 26747 E AndroidRuntime: 	at android.os.Looper.loop(Looper.java:164)
10-28 15:12:24.529 26747 26747 E AndroidRuntime: 	at android.app.ActivityThread.main(ActivityThread.java:6541)
10-28 15:12:24.529 26747 26747 E AndroidRuntime: 	at java.lang.reflect.Method.invoke(Native Method)
10-28 15:12:24.529 26747 26747 E AndroidRuntime: 	at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
10-28 15:12:24.529 26747 26747 E AndroidRuntime: 	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)
10-28 15:12:24.529 26747 26747 E AndroidRuntime: Caused by: java.lang.IllegalStateException: You need to use a Theme.AppCompat theme (or descendant) with this activity.
10-28 15:12:24.529 26747 26747 E AndroidRuntime: 	at android.support.v7.app.AppCompatDelegateImplV9.createSubDecor(AppCompatDelegateImplV9.java:351)
10-28 15:12:24.529 26747 26747 E AndroidRuntime: 	at android.support.v7.app.AppCompatDelegateImplV9.ensureSubDecor(AppCompatDelegateImplV9.java:320)
10-28 15:12:24.529 26747 26747 E AndroidRuntime: 	at android.support.v7.app.AppCompatDelegateImplV9.setContentView(AppCompatDelegateImplV9.java:281)
10-28 15:12:24.529 26747 26747 E AndroidRuntime: 	at android.support.v7.app.AppCompatActivity.setContentView(AppCompatActivity.java:143)
10-28 15:12:24.529 26747 26747 E AndroidRuntime: 	at org.dmfs.tasks.homescreen.TaskListWidgetSettingsActivity.onCreate(TaskListWidgetSettingsActivity.java:47)
10-28 15:12:24.529 26747 26747 E AndroidRuntime: 	at android.app.Activity.performCreate(Activity.java:6975)
10-28 15:12:24.529 26747 26747 E AndroidRuntime: 	at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1213)
10-28 15:12:24.529 26747 26747 E AndroidRuntime: 	at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2770)
10-28 15:12:24.529 26747 26747 E AndroidRuntime: 	... 9 more

Change-Id: Ibdd7ffff6c9db7299c33e057fc48e5d7e058f1b4