Add a KSharedConfigPtr for kcminputrc to KWin::Application

Summary:
There are a few places where KWin needs to read values from kcminputrc.
As I need to add yet another one it makes more sense to properly
structure it like in other cases and have only one kcminputrc hold in
the application. This also allows to better mock the config values in
the integration tests.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D5540