Fix behavior when starting/stopping multiple QML profilers at once

Make sure they're all inserted into the m_startTimes map before any of
them is actually stopped. Otherwise multiple "Complete" messages can be
generated for only one profiling session.

For consistency, also allow all profilers to be started at the same
time with only one call to startProfiling().

Task-number: QTCREATORBUG-11532
Change-Id: I8f80e2f7432c31d5911e139f9632c36f2494cb96
Reviewed-by: Kai Koehne <kai.koehne@digia.com>