note about developing under windows in readme dev
improve preview mode tool layout

while the checkable groupbox is a nice solution, it looks too heavy
on some platforms, e.g. by its bold text. Also the checkbox did not
align with the other checkboxes on many gui styles. Finally I let
the curent gui style decide on the layout margins which looks better.
move all the credits to frescobaldi_app/credits.py
bump version to 2.0.12-dev
edit->select block finally implemented
write much shorter
select block: put cursor at beginning

this is more intuitive; in many cases one wants to extend the selection at the start, e.g. to select a full expression after having selected a block of music that's the argument to a \command.
more elegant

select_block can now use any cursor to perform its trick
fixes a bug when the preference is reset.
Merge pull request #239 from elmamyra/import-export-theme

fixes a bug when the preference is reset.