WaitingThread read some shared memory with the SubProcess
(`mutex`, `m_running`).
When we destroy the SubProcess, we must be sure that WaitingThread has
correctly finished else we may have invalid read/write on freed memory.
We must correctly quote path with space on windows.
This is needed as we can't launch command using a array of string on
windows but by giving only one string using space as separator.
Fixkiwix/kiwix-desktop#268