webui: Add Views panel

[TrimmingFool]

The set of selected labels determines which torrents the user will see. If a user is interested in switching to another view, the labels in State,Labels,Search and Trackers have to be clicked manually.

In order to make switching between views more convenient this PR introduces a Views panel which allows the selection of user-defined views:

Clicking on a view will select all its labels automatically.
A new view can be saved by pressing the [+] button which is only enabled if the view does not exist, already.
Additionally, if the current (manually selected) view includes a saved view, the corresponding row will appear active (similar to when it is clicked).

• There are context menu entries to control the rows in the Views panel:
  • Rename View, Move View -> To top, Up, Down, To bottom, Remove active views
• The circular icon before each row contains the first letter of the view name
• An option in Settings > Format allows to hide the Views panel

Code

• webui: Improve label filtering performance

  • theWebUI.labels is used to speed up the repeated filtering required for the views. (It is a label -> hashes mapping)

• category-panel: Simplify showPanel to updatePanel

  • Sorting of the panels is now done in sortPanels via theWebUI.settings['webui.category_panels'] thus addPaneToCategory does no longer require afterId.

[stickz]

Could we make a new object for panels? The webui object is getting very polluted. Code review is becoming increasingly difficult.

[TrimmingFool]

Could we make a new object for panels?

This sounds like a good idea. I could move the panel methods out of webui.js and into panels.js. However, theWebUI.updateLabels would then belong into panel.js, as well. This would require adjusting some plugins. I think, it might be easier to do this refactoring in a different PR. Let me know, if you prefer it being part of this PR.

[stickz]

Okay, I will merge this pull request into develop. I was going to freeze for version 4.2, but I think it's beneficial to keep going.

It's probably a better idea to refactor into panel.js in a separate pull request.