preferences: use std::string in struct preferences

This is a messy commit, because the "qPref" system relies
heavily on QString, which means lots of conversions between
the two worlds. Ultimately, I plan to base the preferences
system on std::string and only convert to QString when
pushing through Qt's property system or when writing into
Qt's settings.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

export-html.cpp

@@ -23,7 +23,7 @@ int main(int argc, char **argv)
 {
 	QApplication *application = new QApplication(argc, argv);
 	git_libgit2_init();
-	copy_prefs(&default_prefs, &prefs);
+	prefs = default_prefs;
 	init_qt_late();
 
 	QCommandLineParser parser;