Allow for focusable Eww windows

[elkowar]

Currently, Eww windows cannot be focused. This makes widgets like Entry (a text input field) mostly useless, as you cannot type in them without focusing them.

Thus, a config option on <window> should be introduced that allows a widget to be focusable.

Acceptance Criteria

• <window> supports the focusable="true" attribute, which then allows the user to focus the window.
• As a result of that, the input field is usable.

[elkowar]

@alx365 you where waiting for this, no? you can now use the <input> tag properly