Compatibility with ExtJS v. 5.0.0

This is a work in progress PR and it is not yet ready for reviewing or merging.

We currently cannot use the CDN of ExtJS 5 by sencha, as the files do not have an appropriate CORS header. It seems as if this is a problem of me including ExtJS in the wrong way, so there might be a solution.

Anyhow: In order to see the examples working on top of ExtJS 5, one needs to adjust the loading of Ext JS as follows (using a local copy of ExtJS living next to the GeoExt-base folder):

```diff
diff --git a/examples/action/mappanel_with_actions.html b/examples/action/mappanel_with_actions.html
index 97aefb8..051f382 100644
--- a/examples/action/mappanel_with_actions.html
+++ b/examples/action/mappanel_with_actions.html
@@ -4,11 +4,11 @@
     <title>GeoExt2 - Action</title>
 
     <!-- ExtJS -->
-    <script type="text/javascript" src="http://cdn.sencha.com/ext/gpl/4.2.1/examples/shared/include-ext.js"></script>
-    <script type="text/javascript" src="http://cdn.sencha.com/ext/gpl/4.2.1/examples/shared/options-toolbar.js"></script>
+    <script type="text/javascript" src="../../../ext-5.0.0/examples/shared/include-ext.js"></script>
+    <script type="text/javascript" src="../../../ext-5.0.0/examples/shared/options-toolbar.js"></script>
 
     <!-- Shared -->
-    <link rel="stylesheet" type="text/css" href="http://cdn.sencha.com/ext/gpl/4.2.1/examples/shared/example.css" />
+    <link rel="stylesheet" type="text/css" href="../../../ext-5.0.0/examples/shared/example.css" />
     <!-- Basic example styling -->
     <link rel="stylesheet" type="text/css" href="../shared/example.css" />
 
diff --git a/examples/loader.js b/examples/loader.js
index 26a887b..7608409 100644
--- a/examples/loader.js
+++ b/examples/loader.js
@@ -3,6 +3,6 @@ Ext.Loader.setConfig({
     disableCaching: false,
     paths: {
         GeoExt: "../../src/GeoExt",
-        Ext: "http://cdn.sencha.com/ext/gpl/4.2.1/src"
+        Ext: "../../../ext-5.0.0/src"
     }
 });
```

**Current state:**

- [ ] Examples working with either 4.2.1 or 5.0.0 locally
  - [x] Action
  - [ ] MVC-App
    - [ ] uncompiled dev-version
    - [ ] compiled production version
  - [x] Geocoder
  - [x] Feature Grid
  - [x] LayerOpacitySlider
  - [x] Legend Panel
  - [x] Mappanel
  - [x] Permalink
  - [x] Popup
  - [ ] Print extent
  - [ ] Print preview
  - [ ] Feature Renderer
  - [ ] Print extent
  - [ ] Stylegrid
  - [ ] Tree
  - [ ] Treelegend
  - [ ] WFS Capabilities
  - [ ] WMS Capabilities
  - [x] Zoomchooser
  - [x] Zoomslider
- [ ] Testsuite passes with either 4.2.1 or 5.0.0 locally
  - [x] Action
  - [x] container/LayerLegend
  - [x] container/UrlLegend
  - [x] container/WmsLegend
  - [x] container/VectorLegend
  - [x] data/AttributeModel
  - [x] data/AttributeStore
  - [x] data/FeatureStore
  - [x] data/LayerStore
  - [x] data/MapfishPrintProvider
  - [x] data/PrintPage
  - [ ] data/proxy/Protocol
  - [ ] data/reader/Attribute
  - [ ] data/reader/CswRecords
  - [ ] data/reader/WfsCapabilities
  - [ ] data/reader/Wmc
  - [ ] data/reader/WmsCapabilities
  - [ ] data/reader/WmsDescribeLayer
  - [ ] data/WmsDescribeLayerStore
  - [ ] data/ScaleStore
  - [ ] data/StyleStore
  - [ ] FeatureRenderer
  - [ ] Form
  - [ ] form/action/Search
  - [ ] form/Basic
  - [ ] form/field/GeocoderComboBox
  - [ ] form/Panel
  - [ ] grid/column/Symbolizer
  - [ ] Lang
  - [ ] LegendImage
  - [ ] panel/Map
  - [ ] panel/Legend
  - [ ] panel/PrintMapPanel
  - [ ] plugins/PrintExtent
  - [ ] plugins/PrintPageField
  - [ ] plugins/PrintProviderField
  - [ ] selection/FeatureModel
  - [ ] slider/Zoom
  - [ ] slider/LayerOpacity
  - [ ] state/PermalinkProvider
  - [ ] tree/LayerContainer
  - [ ] tree/LayerLoader
  - [x] window/Popup
  - [x] Version
- [ ] Testsuite passes in relevant browsers
  - [ ] recent Chrome
  - [ ] recent Firefox
  - [ ] Internet Explorer 9
  - [ ] Internet Explorer 10
  - [ ] Internet Explorer 11
- [ ] Travis
  - [ ] Use ExtJS 5 also for CI
  - [ ] All tests pass in Travis

I think that makes sense. As not all people will be able to switch immeadiately. up to now, there weren't too many cases where we would need extra code to support both ExtJS4 and 5.