improvements in Debian packaging

 * debian/rules: rename configure script before running dh_auto_configure to make sure that dh_auto_configure automatically passes its buildflags (otherwise i.e. the hardening flags are missing). I guess this should be fixed in the configure script?
 * create notepadqq-common package
 * provide /usr/bin/nqq as symlink to /usr/bin/notepadqq
 * provide /usr/bin/notepadqq via update-alternatives as symlink to /usr/share/notepadqq/notepadqq.sh
 * if notepadqq-gtk is installed, update-alternatives will replace /usr/bin/notepadqq with a symlink to /usr/share/notepadqq-gtk/notepadqq-gtk.sh, which will run notepadqq with '-style=gtk'
 * so if someone like me prefers Qt5 apps with GTK themes, all you have to do is install notepadqq-gtk