enable animations for standalone created buttons

Summary:
--as mentioned at T9899 we are not calling the correct
creator when buttons are created in Standalone mode and
as such animations are disabled when OxygenButton is created
from plasma applets. This patch fixes this and restores
the Standalone behavior mode in order to be identical
with the default one.

Test Plan:
-check that window title bars still work
-check that when buttons are used in a plasma applet
they are animated

Reviewers: #plasma, hpereiradacosta, zzag

Reviewed By: #plasma, zzag

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D16598