Fix several filename and environment variable encoding problems

Fix several filename and environment variable encoding problems

	* src/LaTeX.C
	(LaTeX::deplog): Assume that filenames in log files are stored in
	the file system encoding

	* src/frontends/qt4/qt_helpers.[Ch]
	(internal_path): delete

	* src/frontends/qt4/QGraphics.C: Adjust to change above

	* src/frontends/qt4/QPrefsDialog.C: ditto

	* src/frontends/qt4/QExternal.C: ditto

	* src/frontends/qt4/QInclude.C: ditto

	* src/support/os.h: Document the encoding of filename arguments

	* src/support/os_win32.h: ditto

	* src/support/filetools.C
	(findtexfile): Convert filename from file system encoding

	* src/support/os_win32.C: Convert filenames from utf8 to file system
	encoding and vice versa where needed

	* src/support/os_cygwin.C: ditto

	* src/support/getcwd.C
	(getcwd): Use internal_path() with correct encoding

	* src/support/docstring.[Ch]
	(from_filesystem8bit): new conversion function

	* src/support/environment.C
	(getEnv): convert environment variable from local 8bit encoding to utf8
	(setEnv): convert environment variable from utf8 to local 8bit encoding

	* src/support/environment.h: document encoding of function arguments

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@16753 a592a061-630c-0410-9148-cb99ea01b6c8