Fix vitals monitor UI, use QPainter-based plot for vitals

Create a very simple widget for plotting a graph, and use this instead of the Qwt widget for the vitals monitors. This simplifies usage a bit because it is exactly tailored to our use, and opens up the potential to do interesting things such as having a rolling, rather than scrolling, plot.

Also, remove a lot of unnecessary property setting and use of custom style sheets (which tend to be glitchy) and. In particular, this uses a cleaner grid layout rather than fixed sizes to ensure alignment of various items, and should be more resilient against unusual user settings. Notably, it is a lot less broken on high-DPI displays.