allow zooming on desktop when holding ctrl and scrolling

add prototype of selection mode, span up dashed rectangle

lib/canvas/path/path_transform.dart

@@ -0,0 +1,25 @@
+import 'dart:ui';
+
+import 'ring_number_provider.dart';
+
+extension DashPath on Path {
+  // creates a new dashed path with the given intervals
+  Path dashPath(RingNumberProvider dashArray) {
+    final Path dest = Path();
+    for (final PathMetric metric in computeMetrics()) {
+      double distance = .0;
+      bool draw = true;
+      while (distance < metric.length) {
+        final double len = dashArray.next;
+        if (draw) {
+          dest.addPath(
+              metric.extractPath(distance, distance + len), Offset.zero);
+        }
+        distance += len;
+        draw = !draw;
+      }
+    }
+
+    return dest;
+  }
+}

lib/canvas/path/ring_number_provider.dart

@@ -0,0 +1,13 @@
+class RingNumberProvider {
+  RingNumberProvider(this._vals);
+
+  final List<double> _vals;
+  int _idx = 0;
+
+  double get next {
+    if (_idx >= _vals.length) {
+      _idx = 0;
+    }
+    return _vals[_idx++];
+  }
+}