This adds experimental support for web gui integration so that rclone can fetch and run a web based GUI using the --rc-web-ui and related flags.
It downloads and caches a webui zip file which it then unpacks and opens in the browser.
Introduce stats groups that will isolate accounting for logically
different transferring operations. That way multiple accounting
operations can be done in parallel without interfering with each other
stats.
Using groups is optional. There is dedicated global stats that will be
used by default if no group is specified. This is operating mode for CLI
usage which is just fire and forget operation.
For running rclone as rc http server each request will create it's own
group. Also there is an option to specify your own group.
Before this change serving bucket based objects
`[remote:bucket]/path/to/object` would fail with 404 not found.
This was because the leading `/` in `/path/to/object` was being passed
to NewObject.
Tests have been randomly failing with messages like
listen tcp 127.0.0.1:51778: bind: address already in use
Rework all the test servers so they choose a random free port on
startup and use that for the tests to avoid.
