Fix unused variable warnings

[yetist]

Fix unused variable warnings with G_GNUC_UNUSED.

[lukefromdc]

This suppresses unused variable warnings while keeping the extra variables themselves. Is there a reason such as maintaining ABI compatability with something else for this?

[cwendling]

@lukefromdc each instance should be reviewed, but as those are parameters (not variables) they are part of that function's API. If it's used as a callback or virtual function, the signature has to match what the callers use, whether it's used or not. It's very common in GObject code to have unused parameters because it heavily relies on callbacks and virtual functions, but not all parameters are needed for all cases.

I'm wondering if doing this makes much sense because of this BTW: does this change get rid of everything? And do we want to have G_GNUC_UNUSEDs everywhere, or simply recommend -Wno-unused-parameter for regular builds?

[lukefromdc]

I would suggest the latter: less risk of typos sneaking into code, and normal builders who are not MATE developers do not have any use for these warnings.