Add UI for plugin groups

[Ortham]

This is the UI side of loot/libloot#20. The following needs to be done:

• Add display of a plugin's group to its sidebar entry. The group should be displayed like priority is, and the default group should be omitted to keep it easy to spot non-default values.
• Add a group input to the metadata editor. It needs to be populated with the current value and be a dropdown selector containing the names of currently-defined groups, as only defined groups are valid values. Saving the metadata should also save the group value. If the saved value matches the masterlist value, it should be implicit in the userlist.
• Add a new UI dialog/panel that allows users to define new groups and extend the after sets of existing groups (allowing the removal of entries added in the userlist but not those present in the masterlist).

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Issues to fix:

• Adding a new group doesn't clear the input, so it's possible to add the group multiple times.
• Allow an Enter keypress to add a group when the input is focused.
• Increase the size of edge arrows to make them more visible.
• Improve position of edge arrows, they're currently too close to the source node.
• Clamp zoom so it's not possible to zoom in unnecessarily far.
• Increase the zoom sensitivity so it jumps less between zoom levels.

[Ortham]

As mentioned in the forum thread, I'd ideally like the group editor to be an editable visualisation of the group graph, like this but for now I've implemented it as a couple of lists for simplicity.

EDIT: Here's somthing mostly-functional I've hacked together using Cytoscape:

[Ortham]

Alternative layout using dagre is more logical but less efficiently uses the available space:

[Ortham]

Closing this as it's implemented and merged as of c17c583. I don't expect anything more than tweaks to the functionality at this point.