create a task manager widget

[12rambau]

The idea is to reproduce most but not all of the functionalities of the task manager of the earthengine code editor:

Let's start small and build up from it if there is traction for this functionality:

• [x] #15
• [x] #16
• [x] #17
• [x] #18
• [x] #19
• [ ] #20
• [ ] #21