Better icon for zxtns with no memo

[leto]

Currently it's a white box that almost looks like a broken image.

[oDinZu]

@DenioD I remember you having a icon list, but am unable to find it.

This is also related to #147

For future aid,

L161-178 via txtablemodel.cpp gives an example of adding icons.

[leto]

@csharpee yeah, I think @DenioD did something related to this but I am not sure where the code is