events: allow editing of all used values

1. Allow to change the date/time and
other relevant values of an event
on creation and after it was created.

2. Harmonize layout file names and
variable names.

app/src/main/res/layout/dialog_edit_plain.xml

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="utf-8"?>
+<LinearLayout
+    xmlns:android="http://schemas.android.com/apk/res/android"
+    android:layout_width="match_parent"
+    android:layout_height="match_parent"
+    android:layout_gravity="center"
+    android:gravity="center"
+    android:orientation="vertical">
+
+    <TextView
+        android:id="@+id/dialog_date_picker"
+        android:layout_width="wrap_content"
+        android:layout_height="wrap_content"
+        android:layout_marginTop="20dp"/>
+
+</LinearLayout>