Get rid of some locale dependant functions

Get rid of some locale dependant functions

	* src/output_plaintext.C
	(writePlaintextParagraph): Use compare_ascii_no_case instead of
	compare_no_case, since the strings to compare are pure ASCII anyway.

	* src/frontends/qt4/QPrefsDialog.C
	(setComboxFont): ditto

	* src/MenuBackend.C
	(Menu::checkShortcuts): ditto

	* src/frontends/qt4/QLImage.C
	(QLImage::loadableFormats): Use ascii_lowercase instead of lowercase
	since the input is pure ASCII anyway.

	* src/tex2lyx/text.C
	(splitLatexLength): ditto

	* src/support/lstrings.[Ch]
	(compare_no_case): Get rid of both std::string variants
	(lowercase): Get rid of std::string variant
	(uppercase): Change std::string variant to docstring

	* src/support/tests/regfiles/lstrings: Add new expected output

	* src/support/tests/lstrings.C: Add tests for docstring functions

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@17371 a592a061-630c-0410-9148-cb99ea01b6c8