Fit all digits of the selected input/output value in the text box

This fits the digits from two approaches:

  - wider input boxes
  - fewer digits of precision
Merge branch 'bug-15913-TransferFunctionWidgetsDoNotDisplayAllDigits'

Conflicts:
	Modules/QtWidgetsExt/QmitkColorTransferFunctionCanvas.cpp
	Modules/QtWidgetsExt/QmitkPiecewiseFunctionCanvas.cpp
	Modules/QtWidgetsExt/QmitkTransferFunctionWidget.ui