dev-threads Qt example fork changes

[ClaudioHoffmann]

For our Solum integration, we've decided to just extend the Qt example application. The big feature of our fork will be OpenIGTLink support, but we're also going to add more generally useful changes such as increased feedback and user-friendliness for the GUI. That's why it makes sense to integrate them back into the official repository, and it will also simplify upgrades to new API versions for everyone.
I'm going to push all changes to https://github.com/dev-threads/solum/tree/master as they get developed and describe them in new comments to this PR until it gets closed, so feel free to cherry-pick just the features you want, if any.

The first feature replaces the single-line status bar with a multi-line textbox. Some API and GUI actions result in multiple lines of status text being shown in quick succession, but the current status bar can only show the last one of them. This makes it more difficult than necessary to debug issues, especially during connection to the probe where the generic "connection failed" message previously overwrote the more helpful ConnectionError message. A multiline textbox helps massively there, and can also double nicely as a log.