kcm: use less framey and more conventional style for revert dialog

Typically we only put buttons in footers with Kirigami.OverlaySheet when there is no alternative, typically because the main view is scrollable. In this case, failing to do so would cause content to look cut off at the bottom, so we use a defined footer area.

However that is not a design constraint here, so we can use the nore conventional styling of having the buttons right-aligned at the bottom, not inside a distinct footer area.

Before	After

That extra pading on the bottom appears to be caused by a bug in the base Kirigami.OverlaySheet component itself.

cc @teams/vdg @rizzitello and @carlschwan because I know he prefers the less framey design for this kind of thing 😅