build plugin: merge output- and error tabs, as discussed on reviewboard

Here: https://git.reviewboard.kde.org/r/115323/

With this patch, the build plugin tab now doesn't have an "Output"
and an "Error" tab anymore, but a combined output tab,
where you can switch the "level of detail" using a slider,
from "Full output" to "Only errors".

Alex

original commit by Alex