previewer: refactoring

• Changes preview view function arguments, passes request arguments.

• Sends only file reference from view to plugins.

• Fixes PEP8.

• Adds previewer preference order, settable via CFG_PREVIEW_PREFERENCE, which

is a dictionary such as {'.pdf' : ['pdftk', 'gview']}, specifying that 'pdftk'
it the first choice for PDFs.

Signed-off-by: Adrian-Tudor Panescu <adrian.tudor.panescu@cern.ch>