update IFrame class to ExtJS 7.0 implantation

update IFrame class to ExtJS 7.0 implantation

To get rid of some outdated event handlers like `beforeunload` which
Chromium based browsers are deprecating this year [0].

For those wondering about why we do not use ExtJS implementation
directly here it might be worth adding that the `Ext.ux` name space is
build to a separate file that has a (minified!) size of almost 160
KiB, and we only use a handful of those, so copying is a lot cheaper.

[0]: https://developer.chrome.com/docs/web-platform/deprecating-unload

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>