fix(GUI): restrict webkit drag to header

We ensure that the `-webkit-app-region` attribute is only set to `drag`
on the header element and we explicitly disable it on modals, as this
has unintended behaviour on a non-draggable window with touch-screens.

Change-Type: patch

lib/gui/css/desktop.css

@@ -21,10 +21,14 @@ body {
 
 
 /* Allow window to be dragged from anywhere */
-* {
+body > header {
   -webkit-app-region: drag;
 }
 
+.modal-body {
+  -webkit-app-region: no-drag;
+}
+
 button,
 a,
 input {