create an asset manager widget

[12rambau]

The main objective is of course to reproduce some of the behaviour of the "asset" panel of the GEE code interface:

let's start small and build up from it if there is traction on this feature.

• [x] #7
• [x] #8
• [x] #9
• [x] #10
• [x] #11
• [x] #12
• [x] #13