Hi Dirk,

You can do this using angular interceptors. Read this blog by Matt Brailsford to get the idea : http://24days.in/umbraco-cms/2015/umbraco-7-back-office-tweaks/

For you will want to intercept the content edit view :

That will look something like this :

angular.module('umbraco.services').config([
   '$httpProvider',
   function ($httpProvider) {

       $httpProvider.interceptors.push(['$q','$injector',function ($q,$injector) {
           return {
               'request': function (request) {

                   // Redirect any requests to built in content delete to our custom delete
                   if (request.url === "views/content/edit.html") {
                       request.url = '/App_Plugins/YourPluginFolder/views/content-edit.html';                      
                   }

                   return request || $q.when(request);
               }
           };
       }]);

   }]);

This will intercept all calls to the content edit view and replace the edit view with your own.

In your own you can copy the markup of the built in edit view and add your button. Also you need to change the ng-controller directive to point to your own controller.

Then in your controller you can do this :

angular.module('umbraco').controller('YourControllerName',
    ['$scope', '$controller',
    function ($scope, $controller) {
        // inherit core edit controller
        angular.extend(this, $controller('Umbraco.Editors.Content.EditController', { $scope: $scope }));

        // add your logic for handling button click here
    }]);

The important line here is

angular.extend(this, $controller('Umbraco.Editors.Content.EditController', { $scope: $scope }));

This will make the scope and functions from the core controller available in your controller...like inheriting a class in C#. So all core functionality keeps working.

Only downside is that when you upgrade umbraco it's possible that it breaks because the core view can change.

Dave