qpaeq: add label and convert to dBGain

This is essentially the same pull request in github that Arun Raghavan asked me to switched to gitlab. I added a label to each slider to display the gain value. The biggest change is that I convert the slider's linear coefficient scaling scheme to logarithmic (i.e. in dB). Each slider now range from -30dB to +30dB with 0.5dB increment. Per Arun Raghavan's request, I have broken down my changes to the qpaeq python code into multiple commits. I did tested my qpaeq in my Acer CB3-431 chromebook-converted-to-GalliumOS machine, using python 3 and qt5 libraries. I confirm it is working as intended.