[fix] Get right Column when use custom element replace text content.

[SuperSuperPepper]

If you want to use custom elements replace text content.
example: I want to use custom elements to change some keywords by using the menu, button or other elements for my special feature.
you will get the wrong caret position when you click textview .
because GetVisualColumn doesn't calculate generator elements. so need to recalculate the column.
this PR can fix #416

[siegfriedpammer]

I tested your fix using ICSharpCode.AvalonEdit.Sample which contains the ImageElementGenerator and without your fix clicking right of an inserted image puts the caret into the correct location. With your fix, however, the exact problem occurs that you described in your original issue.

Maybe there is something wrong with your custom element generator? Hard to say, without access to your code. Please take a look at our sample code. Thanks!