Use only one instance of SelectedIds per source

[hanslovsky]

Currently, we have an instance of SelectedIds per source and per mode.This was initially introduced to enforce only one active (selected) id in the merge/split mode. However, having id selection per mode is highly counterintuitive for users. Also, programming logic is much simpler with one SelectedIds per source. I suggest:

• Use only one instance of SelectedIds.
• Introduce a new state, e.g. lastClicked that records the fragment id that was last activated (or Label.INVALID if deactivated)
• use lastClicked to determine which fragment id should be used for merge/split.
• enable multiple active fragments in merge/split mode (maybe unify with highlights mode).
• introduce new highlighting mode for lastClicked, e.g. pulsating between INVALID and actual color

[hanslovsky]

https://github.com/hanslovsky/bigcat/tree/fix-neuron-mesh-generation integrates most of these suggestion with the exception of lastClicked and pulsating highlights that are in a more experimental branch at https://github.com/hanslovsky/bigcat/tree/highlight-last-selection

A pull request, at least for the former, will follow soon.

[hanslovsky]

Fixed with the merge of #65/#85
If there is a need for highlighting the last selection (the last bullet point in the initial comment) we should create a separate issue for that.