Sometimes we want its "remember path" behaviour but it does not make sense to
parent the dialog from main window. When so, caller code no longer needs to
get a reference to a main window.
Also rm last usages of get_parent_main_window().
...\electrum\electrum\gui\qt\qrcodewidget.py:88: DeprecationWarning: an integer is required (got type float). Implicit conversion to integers using __int__ is deprecated, and may be removed in a future version of Python.
qp.drawRect(left+c*boxsize, top+r*boxsize, boxsize - 1, boxsize - 1)
The few other cases that used SimpleConfig.get_instance() now
either get passed a config instance, or they try to get a reference
to something else that has a reference to a config.
(see lnsweep, qt/qrcodewidget, qt/qrtextedit)
