KCM: Make port/profile combobox size dynamic & only show labels when necessary

This MR ensures that the port/profile boxes will fit their content (such that profile names can be read in full), and make their size consistent across delegate instances.

This fixes BUG: 458422

Additionally, the 'Port:' and 'Profile:' labels are hidden unless the port combobox is visible, as there is no need for the label to disambiguate. This saves some width - great for long profile names!

I've not touched the stream items - their comboboxes are long enough to identify each playback device, and the full names of each playback device is already available.