Timeline curve editor: Icons not visible in default theme

The timeline curve editor now works by default in Qt Creator 4.11.1 (since QTCREATORBUG-23353). Unfortunately, the toolbar does not support theming, nor are the icons themeable.

As a minimum, the toolbar should derive from Utils::StyledBar, so that the icons are visible with the default theme. Ideally, the icons were themable, so that a light theme would work.