Provide dialog

[noyainrain]

Make it possible to show a dialog for actions that require additional input.

[noyainrain]

Draft:

/**
 * Modal dialog to query additional information for an action.
 *
 * .. attribute:: result
 *
 *    Promise that resolves with the result of the concluded dialog. ``null`` if the dialog is
 *    closed.
 */
micro.core.Dialog = class extends HTMLElement {};

micro.UI = class {
    /**
     * Active :class:`micro.core.Dialog`.
     *
     * Set to open the given the dialog. May be ``null``.
     */
    get dialog() {}
};