Fix timeline marker in subtitle-editor being obscured

Forces the marker to the front via CSS.

Resolves #858

(Contains #892)

src/main/SubtitleTimeline.tsx

@@ -98,6 +98,7 @@ import { selectTheme } from "../redux/themeSlice";
           ...(refTop.current) && {left: (refTop.current.getElement().clientWidth / 2)},
           top: '10px',
           background: `${theme.text}`,
+          zIndex: 100,
         }}
       />
       {/* Scrollable timeline container. Has width of parent*/}