feature: add CtModifiable#isTransient and related methods

[Artamm]

fix #3651
Implemented modifiers from ModifierKind (Transient, Volatile, Synchronized, Native, Strictfp) for CtModifiable class.

[MartinWitt]

LGTM, nice work!

[Artamm]

LGTM, nice work!

Thank you!

[monperrus]

Hi @Artamm
Thanks a lot for your contribution.

In Spoon we aim at strongly testing all functionalities.

Could you add test cases to specify those new methods?

Thanks!

[Artamm]

@monperrus , thank you for your response.
I added some test cases in ModifiersTest. Maybe I should add another test case/ edit what I did?

[monperrus]

Thanks a lot @Artamm , that's good testing. Do you confirm that we test all those new methods with both "true" and "false"?

[Artamm]

@monperrus, I think yes, these tests are enough to cover all cases for new methods.
Well, I had in mind to add fields in ConcreteClass for additional checks too, but this class extends AbstractClass - so it'll just duplicate the code, IMHO.
If it's needed to add something - let me know, I will add.

[monperrus]

Big thanks for your contribution @Artamm and thanks a lot @MartinWitt for the shepherding