*** Extend preview scripts to also handle PDF output (bug 2165) ***

*** Extend preview scripts to also handle PDF output (bug 2165) ***

* src/PreviewLoader.cpp:
	- add conditions for preview with pdf output

* lib/scripts/legacy_lyxpreview2ppm.py:
	- if no dvi preview file is available, check for a PDF file
	  and process that, if found
	- also produce png output, if requested.

* lib/scripts/lyxpreview2bitmap.py:
	- if no dvi preview file is available, check for a PDF file
	  and process that, if found

*  lib/scripts/lyxpreview_tools.py:
	- move some common functions here

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@25419 a592a061-630c-0410-9148-cb99ea01b6c8