Offer previously defined custom download and upload speed limits

[Karl Richter]

If the user configures a custom download or upload speed limit, the definition has to be repeated every time the limit is set by entering the value in the dialog which is displayed after "Other" is chosen while the pre-defined potentially unused values remain in the popup selection.

The predefined values should be updated based on a most-frequently or most-recently used sorting. If multiple custom limits are used they should evict the pre-defined values if the sorting criteria pushes them up in the list.

experienced with 2.0.0.dev6970+5348465+201703142101~ubuntu16.10.1

[StuffAndThings]

I created #3399 to request this function in gtkui version 2.0.3