port the output kcm from simplekcm to abstractkcm

this has a number of advantages:

• we no longer need to put our fields in the footer
• the QQC2 scollview actually has complete scrollbar coverage on all sides
• the previous point then allows us to use NoWrap to prevent excessively long lines to get wrapped when the format of the output doesn't really need or want them this way (this is particularly useful for a potentially upcoming interrupts kcm where we definitely don't want to line break the format of /proc/interrupts to not make reading unncessarily hard) -- or indeed the KCM can pick any wrap mode it finds best for the given output
• textarea in combination with scrollview does automatically snap to a selection area. this will ultimately make a proper search filed easier to implement

to implement this our loaded state items now always fill the available area and the actual display items are centered inside (e.g. the placeholder message). this also addresses a review concern Nate had because originally we were not able to place the placeholder properly