The prefix tree used by unflatten() (#5836) had a member named slots, which
Qt defines as an empty macro, so including a Qt header before this library
failed to compile. Rename it to table and add a test that includes the
library with Qt's keyword macros (slots, signals, emit, foreach, forever)
defined.
Fixes#5865
Signed-off-by: Niels Lohmann <mail@nlohmann.me>