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 also would prever to support both ExtJS4 and ExtJS5 at least for a couple of time in case we do not have to double a large amount of code or implement siginificant extra code.
I am sure not everybody can switch immeadiately to ExtJS5. 
If it is not possible to support both in one lib, we have to maintain 2 Versions of GeoExt.