Followup for PR #6935, that included a bug.

[ecstrema]

MeasureNumbers and MMRestRanges were deleted in measure::remove,
which caused a crash when undoing the deletion of these elements.

Although this may look as a big refactoring for the small issue it solves, it includes a refactoring of the measure number class into a measureNumberBase class, which MMRestRange, as well as MeasureNumber, inherit. So in fact there's only code moving here.

Previously:   TextBase->MeasureNumber->MMRestRange
WIth this PR: TextBase->MeasureNumberBase->MeasureNumber
              							 ->MMRestRange

This refactoring should lead to a less errorprone implementation.

• I signed CLA
• I made sure the code in the PR follows the coding rules
• I made sure the code compiles on my machine
• I made sure there are no unnecessary changes in the code
• I made sure the title of the PR reflects the core meaning of the issue you are solving
• I made sure the commit message(s) contain a description and answer the question "Why do those changes fix that particular issue?" or "Why are those changes really necessary as improvements?"
• I made sure the commit message title starts with "fix #424242:" if there is a related issue
• I created the test (mtest, vtest, script test) to verify the changes I made

[ecstrema]

updated license and removed unused includes