kdecoration: Increase button size and margins

Improves the visual appearance - makes it look less cramped, and looks more consistent with the new immutable tab design.

It also improves usability as the clickable space is increased for the titlebar and the spacing makes it so it's harder to click the wrong button.

After images are on the left:

I'm unsure of the implications of changing the titlebar margins to qreal - scaling scenarios will probably need further testing but they seem to work fine.

@teams/vdg