Allow modifying the refresh frequency of the remote GEE server

[ricardogsilva]

Introduce new settings (under the advanced section) to allow:

1. Enabling/disabling automatic polling of the remote
2. Configuring the frequency of the refresh operation. It shall make use of a widget that accepts only non-negative integer values, which are to be interpreted as seconds. Make it default to 60, meaning execute a new refresh every minute.

Here is a mockup of how the settings shall look like:

NOTE

Refreshing the list of datasets actually means two different things:

1. Polling the remote server for updates on datasets being generated and possibly initiate their download
2. Scanning the base dir for the presence of new datasets. - this shall not be controllable by the end user. The work described in this issue does not apply here.