Doc: Expand thread technology comparisons

Additions/Changes:
- Add QML's WorkerScript type
- Add QFuture + QFutureWatcher
- Clarify differences between QtConcurrent::run() and the map/filter/
  reduce functions
- Reword table headings
- QThreadPool accepts a priority parameter too (although it's not OS-
  level, unlike QThread)

Rows removed from the table:
- QThread can be "reused" and "task oriented" too, depending on the
  program design. It's hard to convey this in a table though, so I just
  removed it.
- "High level" is ambiguous and doesn't really help readers choose a
  tool to use.

Task-number: QTBUG-33360
Change-Id: Idc5100eaf09033998c155572d44c6c0ad0ba9ef6
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>