Feature Request: Allow plugins to create plugin-specific Dashboards and Widgets

[BenParizek]

The Craft Dashboard makes it easy to add custom Widgets. However, in larger plugins, there can be enough related dashboard items that it is handy to group several widgets together for that specific plugin. Consider Forms or SEO, where there could be several widgets displaying information about each particular plugin alone. We're currently seeing multiple larger plugins start to add dashboards and there are several inconsistencies in how they get implemented.

In these cases, it would be nice to be able to continue to use Craft's Widget API to keep the code and interface familiar, but it would also be nice if those widgets could be created on a "Dashboard" tab of a specific plugin.

I'm imagining something like:

• Extend the Craft Dashboard UI on a custom tab in a plugin
• Allow a user to create widgets from that plugin-specific "Dashboard" tab
• When a widget is created from a tab, there is some way to give it a context (e.g. if I created a widget from My Plugin, the context may be my-plugin, and the widget would only display on that specific plugins dashboard.

This would make it possible to have multiple, similar dashboards on different tabs in Craft, keep a consistent user experience for content editors managing different dashboards, and give plugin developers a consistent API to develop dashboard/widget functionality.

[BenParizek]

Related: #1198