NestedViewerExtension updates.

[petrbroz]

Addressing suggestions from https://github.com/libvarun/forge-extensions/issues/14, this pull request:

• adds a README explaining what the extension does, how it can be used, and how it works
• provides a link to the README in the config JSON
• makes the toolbar button "on/off", with a visual feedback indicating whether the extension is active
• adds a config option to filter only 2d or 3d viewables to be listed in the nested viewer dropdown
• adds a config option to cross-select objects when they're selected in the main or the nested viewer
• makes the nested viewer panel resizable