Make the calendar app embeddable

[LinusCDE]

When done with issue #42.

Would it be possible to have a module that can be either it's own app or embedded like KDE does with it's settings screens? Maybe have the module as a lib (e.g. /opt/lib/libcalendarapp.so) and a binary just open it in "fullscreen" while oxide or other QT apps could directly integrate it.

Originally posted by @LinusCDE in https://github.com/Eeems/oxide/issues/42#issuecomment-698104333