[kstyle] create shadow tiles more explicitly

Summary:
With this changes it's more clear where shadow tiles are created, also it
fixes one possible bug. For example, right now, there are two shady actors
who trigger creation of the shadow tiles:

* the first one lays inside of Style::loadConfiguration(). It creates
shadow tiles as a side effect of configuration of _mdiWindowShadowFactory
* the second one lays inside of ShadowHelper::createPixmapHandles()

Please note, ShadowHelper::createPixmapHandles() is invoked only for X11 not Wayland!

Reviewers: #breeze, hpereiradacosta

Reviewed By: hpereiradacosta

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D11533