[Tasks] Add UI when no tasks are available

[nmetulev]

Proposal: Add UI when no tasks are available in tasks component

Description

The tasks component should have default UI for when no tasks are available to show. The tasks component should also have a template for a developer to provide UI

Rationale

When there are no tasks to show in the task component, there is no ui and some user don't realize there are no tasks

[tramadon]

ic_notasks.svg.zip

[yejuntak]

@sebastienlevert @gavinbarron The current design appear to be align with other Microsoft products. However, please take a look at these two design suggestions above for better user experience. Design on the left holds more interaction with the users, but right aligns with the other MS products design as well as create some sort of indicator that the task is missing.

[sebastienlevert]

This might be something to look at more holistically. I like this for planner tasks, but what about Todo? Files? etc. It might be a good think to even think of a mgt-placeholder component that could be configured with a title, subtitle and icon and being reused across the suite of components we have.