ENH: Slice view annotations

@pieper, @fedorov
This is an implementation of slice view annotations in DataProbe as a part of QIICR Project [1]. It includes the following features:

* Corner text annotations including DICOM annotations.
* Color scalar bar for background volume
* Scaling ruler

The previous pull request with changes can be found at [2].

[1] [QIICR Slicer infrastructure projects](https://github.com/QIICR/ProjectIssuesAndWiki/wiki/Slicer-Infrastructure-Projects)
[2] [Closed pull request](https://github.com/Slicer/Slicer/pull/162)