Add global animation speed slider to workspace options KCM

Summary:
Introduce a new config entry into kdeglobals AnimationSpeed that will
control /all/ animation speeds in kwin and plasma themes and Qt styles
and kirigami.

The value stored is a multiplier.
i.e 1 == normal speed, 0.5 = twice as fast, 2 = twice as slow.
0 is a special value meaning "disable animations"

The QML is a bit complex to represent a logarithmic slider that goes
4x, 2x, 1x, 0.5x, 0.25x, 0.125x

but still allowing a user to type arbitrary values into their config

Test Plan: Looked at config

Reviewers: #plasma, broulik

Reviewed By: #plasma, broulik

Subscribers: ngraham, GB_2, broulik, anthonyfieroni, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D22885