[Maniphest] [Created] T2750: Introduce PreviewJob and OpenJob

[elvisangelaccio (Elvis Angelaccio)]

elvisangelaccio created this task.

TASK DESCRIPTION
  They will take care of extracting files for preview and opening in external applications, respectively.
  This is needed to get rid of `m_tmpOpenDirList` and `m_openFileMode` in `Part`. They are global members and they would lead to race conditions in `Part::slotOpenExtractedEntry()`, if we stop disabling the UI during extraction.

TASK DETAIL
  https://phabricator.kde.org/T2750

WORKBOARD
  https://phabricator.kde.org/project/board/46/

EMAIL PREFERENCES
  https://phabricator.kde.org/settings/panel/emailpreferences/

To: elvisangelaccio
Cc: kde-utils-devel, elvisangelaccio, tctara