WebUI: Allow to control the alternative speed limits

setGlobalDownloadLimit and setGlobalUploadLimit will now modify the
alternative speed limits if they are currently enabled and the regular
speed limits otherwise.

Add also two new commands to toggle the state of the alternative speed
limits and get their current state.

Closes #2203.
WebUI: Use translated strings for speed limits dialogue
WebUI: Add missing comments
WebUI: Show DHT nodes
WebUI: Show connection status
WebUI: Align elements in footer
WebUI: Assign min width to speed limit labels

Try to prevent continuous changes in the width of the labels.
WebUI: Check that all and only the needed parameters are provided

Issue #1560.