Refactor tabs implementation

[isaisabel]

As a user of the ATT&CK Navigator, I want to be able to use the application without significant performance issues as a result of opening many tabs/layers.

The current implementation of the tabs interface has poor performance. Essentially the contents of each tab is its own DOM element, and while these elements are added/removed when tabs are switched for performance reasons, they are separate and the removal/addition of DOM elements can have poor performance. This is likely the cause of #251.

A new system should be developed to change how tabs work. Ideally:

• A data-table is always loaded. There are never multiple data-tables, that single data table will be used for displaying all layers.
• The "new tab" UI and "help" page are moved to modals/dialogs or some otherwise separate system, and are not represented in "tabs". The SVG export interface is likewise moved to a modal/dialog.
• Changing the "tab" changes the current ViewModel (layer) by changing which layer the data-table displays.
  • the layer simply determines the current set of annotations and data-table settings (layout, gradient...). It also sets the domain/version data used to determine the table contents.
  • This may also change the current domain/version according to the domain/version of the layer.
  • Some of this may happen asynchronously since data may have to be loaded when a layer is imported.

Something vaguely like this in terms of data flow:

[isaisabel]

Addressed in #257.