Handing double-colon `:` when completing existing property names in QML

Qt Creator suggests inconvenient / breaking things when changing existing properties in QML: it presents a list of available properties, and inserts them together with extra double-colon which was already there. It results in two double-colons in the code which breaks syntax.

Preferred solution

Detect double-colon on the right side of a cursor (in case of multiple active cursors, detect for each one individually), and don't insert new one if it's already there.