Fix PreImagesRepresentative for group homomorphisms with OnLines action

[ThomasBreuer]

resolves #6241

The PreImagesRepresentative method for IsProjectiveActionHomomorphism claimed to provide enough data in situations where this was in fact not possible:
Arguing with determinants is not enough, one has to ask for the existence of enough scalar matrices, see the example in #6241.

I am not sure about a good strategy for checking the correct conditions,
the membership test for a generating scalar matrix is new and is perhaps more expensive than necessary.
On the other hand, the old code computed the kernel of the homomorphism.

Some additional tests might be interesting.
They should mainly check that the changed method returns fail.

[ThomasBreuer]

(At least) one of the tests gets stuck, due to the newly introduced membership test.
This is the test from testbugfix/2016-03-03-t00332.tst which had been added by @hulpke.
Perhaps he can tell me why we are allowed to assume that the old code for LinearActionBasis is correct in this example, such that we can compute preimages under the projective action without NiceMonomorphism overhead.