Make file downloads cancellable

lib/screen/pages/files/fileElement.dart

@@ -52,6 +52,7 @@ class FileElement extends StatefulWidget {
 
 class _FileElementState extends State<FileElement> {
   double percent = 0;
+  Future<DownloaderCore>? downloadCore;
 
   Widget getSubtitle() {
     if(widget.file.currentlyDownloading) {
@@ -89,13 +90,40 @@ class _FileElementState extends State<FileElement> {
             },
           ));
         } else {
+          if(widget.file.currentlyDownloading) {
+            showDialog(context: context, builder: (context) {
+              return AlertDialog(
+                title: const Text("Download abbrechen?"),
+                content: const Text("Möchtest du den Download abbrechen?"),
+                actions: [
+                  TextButton(onPressed: () {
+                    Navigator.of(context).pop();
+                  }, child: const Text("Nein")),
+                  TextButton(onPressed: () {
+                    downloadCore?.then((value) {
+                      if(!value.isCancelled) value.cancel();
+                      Navigator.of(context).pop();
+                    });
+                    setState(() {
+                      widget.file.currentlyDownloading = false;
+                      percent = 0;
+                      downloadCore = null;
+                    });
+                  }, child: const Text("Ja, Abbrechen"))
+                ],
+              );
+            });
+
+            return;
+          }
+
           setState(() {
             widget.file.currentlyDownloading = true;
           });
 
           log("Download: ${widget.file.path} to ${widget.file.name}");
 
-          FileElement.download(widget.file.path, widget.file.name, (progress) {
+          downloadCore = FileElement.download(widget.file.path, widget.file.name, (progress) {
             setState(() => percent = progress);
           }, (result) {
             if(result.type != ResultType.done) {