Merge branch '13-add-help-symbol-on-first-top-cell' into 'master'

Resolve "Add help symbol on first top cell"

Closes #13

See merge request !17

android/gradle.properties

 org.gradle.jvmargs=-Xmx1536M
 android.useAndroidX=true
 android.enableJetifier=true
-app.versionName=0.0.16
-app.versionCode=16
+app.versionName=0.0.17
+app.versionCode=17

lib/entities/cell.dart

@@ -10,15 +10,37 @@ class Cell {
     @required this.value,
   );
 
-  Container widget(Data myProvider, int row, int col) {
+  Widget widget(Data myProvider, int row, int col) {
     String imageAsset = this.getImageAssetName(myProvider);
 
-    return Container(
-      child: Image(
+    Image cellColorImage = Image(
       image: AssetImage(imageAsset),
       fit: BoxFit.fill,
       key: ValueKey<int>(imageAsset.hashCode),
+    );
+
+    if ((row == 0) && (col == 0)) {
+      return Stack(
+        alignment: Alignment.center,
+        children: <Widget>[
+          cellColorImage,
+          Center(
+            child: Text(
+                '•',
+                style: TextStyle(
+                  color: Colors.black,
+                  fontSize: 30.0,
+                  fontWeight: FontWeight.w800,
+                ),
+                textAlign: TextAlign.center
               ),
+          ),
+        ]
+      );
+    }
+
+    return Container(
+      child: cellColorImage,
     );
   }