While reloading a text document, the bookmarks got lost since their
associated QTextBlocks were deleted.

This patch makes sure that before reloading, the bookmarks are removed
non-persistently in the same way as when closing a document, and that
they are restored after the document was reloaded.

Currently, no effort is made to update the location of the bookmarks
based on the way the file changed.

Task-number: QTCREATORBUG-1281
Reviewed-by: dt