kwin_bridge: Load DecorationButton without the "button" keyword

Plugin keywords have been deprecated. Breeze and Oxygen no longer use
the "button" keyword when registering their button plugins, so loading
them now fails and blank assets get generated.

Attempt loading DecorationButton without using the keyword, and if this
fails, try the deprecated keyword like in the KWin commit 6f110bca.

BUG: 442659


(cherry picked from commit c10dff60)