Resolve "Versioning of tests"

Closes #15 (closed)
An estimated test duration can now be added to an test case version
The first and last sut version which can be tested with a test case version can be added to the test case version. The last version has to be newer than the first version.

Edit:
A test case can now be duplicated with the same history as the currently edited test case. The versions that are newer than the currently edited are deleted.