Accessibility improvements

[szimek]

• [x] add lang attribute to html element
• [ ] add role and ESC key handling to modal-dialog component
• [ ] lock focus inside modal-dialog component when it's visible
• [ ] lock scroll when modal-dialog component is visible
• [ ] add role and key handling to popover-confirm component
• [ ] lock focus inside popover-confirm component when it's visible (what if there are multiple popovers visible at the same time, e.g. when receiving files from multiple peers at the same time?)
• [ ] announce start of bundling (in case of multiple files)
• [ ] announce start of sending
• [ ] announce start of receiving
• [ ] announce end of sending
• [ ] announce end of receiving
• [ ] announce progress of 1) bundling, 2) receiving and 3) sending transfer to let users know that something is happening
• [ ] announce errors