Fix the last couple of sorting bugs (#77)

This fixes the following bugs:
- Sort category is forgotten after lock/unlock
- The sort mode is not respected for new entries

I got a little carried away while working on this patch and also included the
following other enhancements:
- Simplify the SortCategory, Theme and ViewMode enums
- Simplify usage of string resources
- Don't call notifyDataSetChanged and runLayoutAnimation unnecessarily

app/src/main/java/com/beemdevelopment/aegis/Theme.java

@@ -5,43 +5,13 @@ public enum Theme {
     DARK,
     AMOLED;
 
+    private static Theme[] _values;
+
+    static {
+        _values = values();
+    }
+
     public static Theme fromInteger(int x) {
-        switch (x) {
-            case 0:
-                return LIGHT;
-            case 1:
-                return DARK;
-            case 2:
-                return AMOLED;
-        }
-        return null;
-    }
-
-    public static int getThemeNameResource(int x) {
-        switch (x) {
-            case 0:
-                return R.string.light_theme_title;
-            case 1:
-                return R.string.dark_theme_title;
-            case 2:
-                return R.string.amoled_theme_title;
-        }
-        return R.string.light_theme_title;
-    }
-
-    public static String[] getThemeNames() {
-        return new String[]{
-                "Light theme",
-                "Dark theme",
-                "Amoled theme"
-        };
-    }
-
-    public static int[] getThemeNameResources() {
-        return new int[] {
-            R.string.light_theme_title,
-            R.string.dark_theme_title,
-            R.string.amoled_theme_title
-        };
+        return _values[x];
     }
 }