Write animation speed to kdeglobals

When the animation slider moved to look and feel a UI was kept within
the compositing KCM that was only visible for non Plasma users so they
still had a way to access this.

This non-plasma version still wrote to kwinrc. In theory this was fine
unless you used both. We also hit an issue where a stray
m_settings->save() call in `reenableGl` would sync the settings to the
wrong place.

This patch moves everything to write to kdeglobals and cleans up any old
entries here.

BUG: 431259
(cherry picked from commit 0bb3eb2b)