Fixes #952
Fix problem with to early SpeedUp
Now it is possible to use custom StringFormat ( "{}{0:N0} pcs." ) in XAML other than the standard ones (C,E,N,F,G)
Changed checking for _valueTextBox in OnValueChanged.
Updated XML-documentation
Some optical improvements
CaretIntex is now set to last position if +/- is pressed
[Fix] Delay property was not correctly working
[Fix] StringFormat did not worked anymore when using "N0" but worked when using "{}{0:N0}".
Merge pull request #953 from xxMUROxx/hotfix/Issue#952

NumericUpDown: Fixes #952 and improve stability