Creates thumbnail from marker's photos for marker's list.

These thumbnails are created and loaded async using cache from free memory available.
Fixes #545.

src/main/java/de/dennisguse/opentracks/adapters/ScrollVisibleViews.java

@@ -0,0 +1,42 @@
+package de.dennisguse.opentracks.adapters;
+
+import android.view.View;
+import android.widget.AbsListView;
+
+import androidx.annotation.NonNull;
+
+/**
+ * AbsListView.OnScrollListener class that can be used to know what views in a ListView are currently visible while scrolling.
+ */
+public class ScrollVisibleViews implements AbsListView.OnScrollListener {
+    private int from = -1;
+    private int to = -1;
+
+    private VisibleViewsListener visibleViewsListener;
+
+    public ScrollVisibleViews(@NonNull VisibleViewsListener visibleViewsListener) {
+        this.visibleViewsListener = visibleViewsListener;
+    }
+
+    @Override
+    public void onScrollStateChanged(AbsListView view, int scrollState) {
+        if (scrollState == AbsListView.OnScrollListener.SCROLL_STATE_IDLE || scrollState == AbsListView.OnScrollListener.SCROLL_STATE_FLING || scrollState == AbsListView.OnScrollListener.SCROLL_STATE_TOUCH_SCROLL) {
+            if (from >= 0 && to >= 0) {
+                for (int i = from; i < to; i++) {
+                    View viewChild = view.getChildAt(i - from);
+                    visibleViewsListener.onViewVisible(viewChild, i);
+                }
+            }
+        }
+    }
+
+    @Override
+    public void onScroll(AbsListView view, int firstVisibleItem, int visibleItemCount, int totalItemCount) {
+        from = firstVisibleItem;
+        to = firstVisibleItem + visibleItemCount;
+    }
+
+    interface VisibleViewsListener {
+        void onViewVisible(View view, int position);
+    }
+}