qt4 subplottool is modal now + new layout
subplottool -> whats_new.rst
fix wrong backend name in whats_new
cleanup in subplottool + new style qt fix

Signed-off-by: Thomas Hisch <t.hisch@gmail.com>
fix typo in furmsubplottool

Signed-off-by: Thomas Hisch <t.hisch@gmail.com>
Merge pull request #2594 from rhoef/newsubplottool

New layout for qt4 subplottool + QMainWindow -> QDialog