Java: Add common base class for SwitchStmt and SwitchExpr

**Description of the issue**
Would it be possible to add a common base class for SwitchStmt and SwitchExpr?
Currently both classes define common predicates and it is often useful to find out whether a switch covers certain values, regardless of whether it is a statement or an expression.
Common predicates:
- `getACase`
- `getAConstCase`
- `getAStmt`
- `getDefaultCase`
- `getExpr`
- `getStmt`

Some built-in QL types and queries would likely profit from this as well (though classes like `ConstSwitchStmt` would have to be redesigned to extend from this new class?).