New Channels list interface

[ichorid]

To display 1 000 000 000 torrents in a single channel we have to redesign our Channels interface.

First of all, @devos50's lazy loading routine expects works only on complete lists of items. I.e. if a channel has 1M entries, REST endpoint will send all 1M entries in a single REST response and then that list will reside in GUI's memory. This is a purely technical problem.

Second, the channels interface we have now is beautiful, but useless for viewing/managing channels that has more than a few dozen torrents in it. To fix that we have to copy some features that torrent trackers employ to manage large collections of torrents, namely:

• Table view
• Substring-based real-time filtering
• Sorting based on various criteria
• Hierarchical view (nested channels target)
• Inline editing of metadata attributes (title and tags)

To achieve this, we'll have to create a proper QT MVC-style table widget that would implement lazy loading by dynamically sending REST API calls to Tribler core.

[ichorid]

solved by #4090