This changes current behavior while opening links. Link is now opened in
next split by default. If you use CTRL+Click to open links, it will also
open in next split. However, by using CTRL+ALT+click it will open in
current split.

There are two new checkboxes in Tools/Options/Text Editor/Display:
- "Open Links in New Split" - if it is checked, links will not be opened
in current split. However, if document with link is already opened, it
will be used to open the split
- "Force open links in next split" - Links will always open in next
split, even if their document is already opened somewhere else.

Task-number: QTCREATORBUG-8117
Change-Id: Ib99075b55d9e9683ed2c2386767227457de0a3fc
Reviewed-by: David Schulz <david.schulz@digia.com>