gui: Avoid using HAVE_QSCINTILLA in more header files. * gui-settings.h, settings-dialog.h: Don't include QScintilla header. Forward-declare QSciLexer class instead if necessary. Declare all member functions unconditionally. * gui-settings.cc (gui_settings::get_valid_lexer_styles, gui_settings::read_lexer_settings), settings-dialog.cc (settings_dialog::update_lexer, settings_dialog::get_lexer_settings, settings_dialog::write_lexer_settings): Define functions unconditionally. * gui-preferences-ed.h: Don't include QScintilla header. Remove definition of local variable os_eol_mode from header. * gui-preferences-ed.cc (os_eol_mode): Move definition of local variable here.