Move most common code into Qt5Instance (static) functions.
Not much saved with regard to LOC, but since the code was
95% C'n'P, I hope it'll be less error prone this way.

Also handle all QApplication argument pointers via
std::unique_ptr from the beginning.

Change-Id: I87b2f571d398db7ccce3e740f16205b251be4ced
Reviewed-on: https://gerrit.libreoffice.org/69578
Tested-by: Jenkins
Reviewed-by: Aleksei Nikiforov <darktemplar@basealt.ru>