[Qt] add all used colors in txtablemodel to guiconstants

- add colors used in TX status decoration
[Qt] simplify return code and return values in txtablemodel

- also move an added space in a string where it belongs
Merge pull request #4667

bbad683 [Qt] simplify return code and return values in txtablemodel (Philip Kaufmann)
21f1516 [Qt] add all used colors in txtablemodel to guiconstants (Philip Kaufmann)