Add Target property to most widgets

[ralphocdol]

This is mainly for users who use Glance as a New Tab page.

Addresses #870, #696

For Title or Group title URL

- type: some-widget # a valid one of course
  title: Some Widget
  title-url: https://some-url.com
  title-target: _self

_{is title-url-target better?}

For most widgets

- type: markets
  target: _self
  markets:
     - symbol: SPY
       name: S&P 500

Monitor and Docker Per Item Target

- type: monitor
  target: _self
  sites:
     - title: Site 1
       url: https://site-1.com
       target: _blank
     - title: Site 2
       url: https://site-2.com

This deprecates the SameTab of docker, but it should still work.