[8.x] Add qualifyColumns method to Model class

[theVannu]

Hi All,
this is my first pull request, I hope all is right.
I would like to suggest to add a qualifyColumns (note the "s") method to the Model class in order to qualify multiple column at once, avoiding to call qualifyColum method multiple times.
I think this may help when we need to qualify multiple columns when using select method of Builder.

1. without qualifyColumns:

$query->select( $userModel->qualifyColumn('name'), $userModel->qualifyColumn('surname'), $userModel->qualifyColumn('address') );

2. with qualifyColumns:

$query->select($userModel->qualifyColumns(['name', 'surname, 'address']));

[driesvints]

Looks like tests are failing. Please fix these and then mark the PR as ready for review. Thanks