oxygen: Replace action button SVG with the more appropriate KDE4 original

Some mistake must have happened at some point when migrating the Air and Oxygen desktop themes because Air's 'actionbutton.svg' was copied over to Oxygen.

The two should not share an identical design for this button because this SVG is white, and Oxygen is a dark desktop theme. There is also too much blue glow, which fits with Air.

Scraping original KDE4 files confirms this - Oxygen had a distinct, dark action button SVG.

This MR restores that old SVG in order to improve theme consistency.

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