View / add support for viewport extent constraint

This introduces a breaking change:

The `extent` view option now constrains the whole viewport and not just the
view center.
The option `constrainOnlyCenter` was added to keep the previous behaviour.

Constraining the whole viewport and not only the view center means
that the center and resolution constraints must be applied with a knowledge of
the viewport size.

examples/extent-constrained.js

@@ -0,0 +1,25 @@
+import Map from '../src/ol/Map.js';
+import View from '../src/ol/View.js';
+import TileLayer from '../src/ol/layer/Tile.js';
+import OSM from '../src/ol/source/OSM.js';
+import {defaults as defaultControls} from '../src/ol/control/util';
+import ZoomSlider from '../src/ol/control/ZoomSlider';
+
+const view = new View({
+  center: [328627.563458, 5921296.662223],
+  zoom: 8,
+  extent: [-572513.341856, 5211017.966314,
+    916327.095083, 6636950.728974]
+});
+
+new Map({
+  layers: [
+    new TileLayer({
+      source: new OSM()
+    })
+  ],
+  keyboardEventTarget: document,
+  target: 'map',
+  view: view,
+  controls: defaultControls().extend([new ZoomSlider()])
+});