UPDATE: I didn't like the odd shapes of the raindrops in my first attempt, directly below. So I changed the way I created the random shapes, by using a simple hard round brush to create a defined brush set, then turned on the shape dynamics, scattering and smoothing brush presets, to create the rounder shapes. I also filled the window pane with tiny droplets as well as the scattering of larger droplets. This makes the pane look wetter and more realistic.
Here is the update:
