Study: Improve DocIndex component performance

[mkoskim]

With quick testing, the main performance bottleneck in the editor view is the index of the active section. Non-active indices do not hurt the performance. The problem: because index is for Drag'n'drop, it needs to be mostly up-to-date. We might figure out some clever solutions to improve its performance.

Source code:

https://github.com/mkoskim/mawejs/blob/master/src/gui/common/docIndex.js

Some ideas:

• We could try to make non-DnD'able index and check, if the performance bottleneck comes from dnd-enabled index rendering.
• We could try if the main problem is rendering the active container (the one where the cursor, that is, selection is).