Overhaul ui: small improvements

Some very small improvements for overhauling the job view:

• use an enum class instead of a simple enum and ints to make things clearer
• move JobView.* to GUI/Views/JobWidgets

requested review from @m.puchner

assigned to @m.puchner

approved this merge request

assigned to @t.knopff and unassigned @m.puchner

mentioned in commit b16b7f7c

merged