Performence tests

[wmww]

Would be nice to be able to measure performance, especially of the GDB backend

[wmww]

Ideally we'd have a fancy automated solution, but until then we can use this:

# Run QTerminal as a Wayland app inside GDB. Quite immediately and measure how long it takes.
QT_QPA_PLATFORM=wayland time wldbg -g --ex r --ex q --args qterminal -e 'sleep 0.1 && exit'

Time's output doesn't have newlines for some reason but it's readable. On my XPS (bunch of Qt debug symbols are installed, might have an impact) it reports 33.56 in user time, compared to just 5.57 when run in non-wayland debug GDB. #37 probably related.