Add options to show or hide New Tab Page widgets

[rebron]

Test plan

See #4510 (comment)

Description

Provide users with the ability to show/hide the individual elements on the New Tab page: Brave Stats, Clock, Top Sites.

Designs

Hover or tab over the Brave Stats widget to highlight it and display an icon representing menu options:

Clicking/selecting the icon shows a menu with widget options:

Hover or tab over the Clock widget to highlight it and display an icon representing menu options:

Clicking/selecting the icon shows a menu with widget options:

Hover or tab over the Top Sites widget to highlight it and display an icon representing menu options:

Clicking/selecting the icon shows a menu with widget options:

The user can add widgets back to new tab page via the Dashboard Settings menu:

When the user has removed a widget, its corresponding toggle in the Dashboard settings menu is toggled off. This is an example of how the Dashboard would look with everything toggled off:

Design Specs

The user can right click anywhere on the widget to bring up the same options menu:

[imptrx]

Test Plan:

1. Open new tabs page with a fresh profile - all NTP elements should appear
2. Open settings menu from the settings icon - 3 new settings should exist below show background
3. Toggle the three options; show clock; show stats; show top sites - they should disappear/reappear on the page
4. Settings should persist after browser is closed and reopened

[btlechowski]

Awesome!

Verification passed on

Brave	0.69.117 Chromium: 76.0.3809.100 (Official Build) beta (64-bit)
Revision	ed9d447d30203dc5069e540f05079e493fc1c132-refs/branch-heads/3809@{#990}
OS	Ubuntu 18.04 LTS

Verified test plan from #4510 (comment)

Verification passed on

Brave	0.69.121 Chromium: 76.0.3809.132 (Official Build) beta (64-bit)
Revision	fd1acc410994a7a68ac25bc77513d443f3130860-refs/branch-heads/3809@{#1035}
OS	Windows 7 Service Pack 1 (Build 7601.24494)

Verified test plan from #4510 (comment)

Verified passed with

Brave	0.69.123 Chromium: 76.0.3809.132 (Official Build) beta (64-bit)
Revision	fd1acc410994a7a68ac25bc77513d443f3130860-refs/branch-heads/3809@{#1035}
OS	Mac OS X

Verified test plan from #4510 (comment)