The goal of task T916 is to stop disabling the Part UI while extracting an
entry. To do so, we need first to make sure that no race condition would occur
if extracting two or more entries in parallel.

A race condition might occur in Part::slotOpenExtractedEntry(), where
m_openFileMode is a global member that could be accessed concurrently between
different threads. We can stop using this variable if we introduce dedicated
jobs for preview/opening of files. This way we can simply do a dynamic_cast on
the local job variable, to check whether the user wants to open or open-with
the file.

Preview is totally unrelated so it can be moved into a different slot. We can
also make sure that the PreviewJob deletes the temporary directory upon
completion.

Differential Revision: D1749