Standardize setting of level window values

Summary:
Solves T29213 . In the level window preset dialog, I changed the Spinboxes to DoubleSpinboxes to enable inputting of floating point values. While at it, I noticed that the "change range" action in the level window context menu already allows floating point values, but uses a precision of 10 digits. For uniformity and readibility I reduced this, so both input methods now use a precision of 2 digits behind the comma.
At the moment, the values shown below the level window slider do not show floating point values. One could add this functionality in QmitkLineEditLevelWindowWidget::OnPropertyModified(), where the string is ensured to show an integer. However, this would cluster up the UI a bit, since the textbox is quite slim, so in most cases more (unnecessary) digits would only reduce visibility. Since behind the scenes the correct (floating point) value is used, I think it is best to leave this as it is.

Test Plan: See T29213 . Enter the level window context menu (right click on the level window slider) and try adding a preset or changing an existing one to include floating point values. Also try changing the range using "change range". Both methods should allow two digits behind the comma of floating point precision.

Reviewers: O3 MITK Reviewer Group II, kislinsk

Reviewed By: O3 MITK Reviewer Group II, kislinsk

Subscribers: kislinsk

Differential Revision: https://phabricator.mitk.org/D678