ref: move all prop type declaration to flow

For the most part the changes are taking the "static propTypes" declaration off
of components and declaring them as Flow types. Sometimes to support flow some
method signatures had to be added. There are some exceptions in which more had
to be done to tame the beast:
- AbstractVideoTrack: put in additional truthy checks for videoTrack.
- Video: add truthy checks for the _videoElement ref.
- shouldRenderVideoTrack function: Some component could pass null for the
  videoTrack argument and Flow wanted that called out explicitly.
- DisplayName: Add a truthy check for the input ref before acting on it.
- NumbersList: Move array checks inline for Flow to comprehend array methods
  could be called. Add type checks in the Object.entries loop as the value is
  assumed to be a mixed type by Flow.
- AbstractToolbarButton: add additional truthy check for passed in type.

react/features/remote-control/components/RemoteControlAuthorizationDialog.js

@@ -1,6 +1,5 @@
 // @flow
 
-import PropTypes from 'prop-types';
 import React, { Component } from 'react';
 import { connect } from 'react-redux';
 
@@ -10,50 +9,46 @@ import { getParticipantById } from '../../base/participants';
 
 declare var APP: Object;
 
+/**
+ * The type of the React {@code Component} props of
+ * {@link RemoteControlAuthorizationDialog}.
+ */
+type Props = {
+
+    /**
+     * The display name of the participant who is requesting authorization for
+     * remote desktop control session.
+     */
+    _displayName: string,
+
+    /**
+     * Used to show/hide the dialog on cancel.
+     */
+    dispatch: Function,
+
+    /**
+     * The ID of the participant who is requesting authorization for remote
+     * desktop control session.
+     */
+    participantId: string,
+
+    /**
+     * Invoked to obtain translated strings.
+     */
+    t: Function
+};
+
 /**
  * Implements a dialog for remote control authorization.
  */
-class RemoteControlAuthorizationDialog extends Component<*> {
-    /**
-     * RemoteControlAuthorizationDialog component's property types.
-     *
-     * @static
-     */
-    static propTypes = {
-        /**
-         * The display name of the participant who is requesting authorization
-         * for remote desktop control session.
-         *
-         * @private
-         */
-        _displayName: PropTypes.string,
-
-        /**
-         * Used to show/hide the dialog on cancel.
-         */
-        dispatch: PropTypes.func,
-
-        /**
-         * The ID of the participant who is requesting authorization for remote
-         * desktop control session.
-         *
-         * @public
-         */
-        participantId: PropTypes.string,
-
-        /**
-         * Invoked to obtain translated strings.
-         */
-        t: PropTypes.func
-    };
-
+class RemoteControlAuthorizationDialog extends Component<Props> {
     /**
      * Initializes a new RemoteControlAuthorizationDialog instance.
      *
      * @param {Object} props - The read-only properties with which the new
      * instance is to be initialized.
      */
-    constructor(props) {
+    constructor(props: Props) {
         super(props);
 
         this._onCancel = this._onCancel.bind(this);