is it possible to assign the metronome (start/stop) to a midi button?

You can assign almost any parameter to a midi controller.
There are 8 drumpads on my midikeyboard which i don't use so i assigned them to the metronome, record, play and a few other PTQ functions.
You can right click on most of PTQ's buttons and sliders to assign a midi controller. Unfortunately this doesn't apply to the metronome.

Follow these steps to make it work:

In order to assign a controller button to a certain function, open PTQ and go "Edit"-->"Preferences" and switch to the "MIDI" tab.

Click on "Insert" (bottom, right). A new entry in the mapping list appears.
1. Click on the first field which sais "Invalid" and choose Control Change.
(If you don't know the particular controller, just press the button on your midi device you want to assign something to. The midi monitor on the upper half shows the controller number of the button or pad you just pressed)
2. In the second field choose the midi channel on which the device transmits data. If you're not sure (or have only one controller connected) choose "any".
3. In the third field choose a function. In your case "MIDI sequencer (standalone)"-->"Toggle metronome"
Done.

Don't forget to save your mapping and make sure to click on "set as default".

It is also possible to assign the functions "Slower metronome" and "Faster metronome" to two additional buttons on your device.
They are also found under "MIDI sequencer (standalone)".