Selection on fragmented scope

[michalstocki]

When we enable selection on many paragraphs at once, we should't allow to create selection between several paragraphs - it may contain other paragraphs' siblings.

$('p').customSelection();

The problem is wider. We should ensure that always only one pair of markers is visible at a time, even if we attach custom selection on contents of different windows (e.g. iframe's). It also means that every document-connected operation (e.g document.createElement()) should be executed in context of window containing selected element.