add a fallback gen_version.h if for some reason the creation of the file fails
PreferencesDialog: New settings for font size of SQL editor and log

Add two new settings to allow changing the font size inside the SQL log
and the SQL editor.

Increase the default font size from 8 to 9 - better a bit too big on
some systems than too small on others.

Also do some restructuring in the MainWindow to avoid duplicate code to
reload settings.