[platforms] Call setSoftWareCursor in init instead of ctor

Summary:
Platform::setSoftWareCursor creates connections to the Cursor in order
to trigger repaints whenever the cursor position changes. The Cursor is
created before Platform::init is called, but after the Platform is
created. Thus the call needs to happen in init, otherwise the cursor
is not rendered correctly.

BUG: 356328
FIXED-IN: 5.7.0

Reviewers: KWin, Plasma on Wayland

Subscribers: plasma-devel, kwin

Tags: Plasma on Wayland, KWin

Differential Revision: https://phabricator.kde.org/D2025