Clarified usage of DivisionalIndex

[oleg68]

This is the first PR related to #1787

GrandOrgue supports two types of divisionals combinations:

• a manual ("old style") divisional. GODivisionalCombination::m_IsSetter == false
• a setter element ("banked") divisional. GODivisionalCombination::m_IsSetter == true

This PR

• renames GODivisionalCombination::m_DivisionalNumber to m_DivisionalIndex, because if it is a setter element, it is an index including the bank number
• changes it's type to unsigned
• moves it's initialising from Init() and Load() to the constructor. It is essential that it is set together with m_IsSetter
• sets GODivisionalCombination::m_IsSetter to true in all GODivisionalSetter combinations.

It is just refactoring. No GO behavior should be changed.

[oleg68]

I thought that this approach is wrong. The right way is not to store divisionalindex in the combination, but set in on the button or the setter level.