Add const modifiers to function parameters and functions

Rationalisation of functions and function parameters: make function
parameters const/const reference and make functions const, as
appropriate.

N.B. We decided _not_ to constify setters in CupsCtrl, since even
though technically these are const functions within the CupsCtrl
class, they do make changes beyond that class in CUPS itself.
Merge branch 'tsc/add_const_modifiers' into merge_const_modifiers

Conflicts:
	wmlcups/CupsCtrl.h
	wmlcups/CupsCtrlQueue.cpp