Merging of text-cursors

[kallisto56]

Hi there,

This pull-request changes behaviour of text-cursors:

• Multiple carets sitting on the same position will be merged 56 vs 56;
• Caret will be merged if it's within range of the selection 56 vs 0-56;
• Selections will be merged only if they overlap 0-56 vs 0-112;
• As stated above, selections that go one after another, will not be merged 0-56 vs 56-112;

Reason for closing previous PR was that I went in my own way, that differed from both VSCode and Sublime Text. This implementation used VSCode as a reference, which felt more correct.