Make QtWebEngine dependency optional

The QtWebEngine dependency should be optional, for several reasons:

• so FSDG distros that don't package QtWebEngine due to freedom concerns can still build and provide client-qt;
• making QtWebEngine an optional dependency could make cross builds more feasible/doable again;
• makes it much more feasible/practical to build client-qt for smaller devices/platforms like the Pinephone and/or embedded devices where it would take an eternity or just not be possible to build QtWebEngine; and
• potentially let us build libqt-jami statically and on one (older) distro for each package system instead of 10 or more builds of libqt-jami, one for each distro.