Finally! After 3.5 years, GIMP 2.8 is out, with the much desired single-window mode (you can activate it in the “Windows” menu) and many other new features: https://www.gimp.org/release-notes/gimp-2.8.html
At the moment you can only download the source code from the official site, and then you must compile the software by yourself. However, if you are on Linux you can use the unofficial repository ppa:otto-kesselgulasch/gimp by typing in the terminal:
sudo add-apt-repository ppa:otto-kesselgulasch/gimp
sudo apt-get update
sudo apt-get install gimp
If you are upgrading from an older version of GIMP, rather than installing from scratch, instead of the last of the three lines above, type the following:
sudo apt-get dist-upgrade
Enjoy!