Create commands for remembering and restoring the tab and focused element

[david-tejada]

This would be useful to use in conjunction with the vscode draft functionality. One of the issues that arises when using this functionality is that you open another tab and when you go to submit it fails because the tab where you originally use the draft command is not the active one.

The commands could be something like this:

• rememberActiveElement: Stores the tab and active element in that tab.
• restoreActiveElement: It makes the stored tab the active one and restores the focus to the proper active element.

I need to take into account that the active element could be in a frame different than the main one. I think the simplest approach is to send the command to all the frames but in the frame only store the active element if it's not an iframe element. When the command to restore the active element is issued I again send the command to all frames but only restore the focus if the storedActiveElement is not undefined.

I also need to take into account if the active element is a shadow host to retrieve the active element of the shadow root.