bringing back the triangle (partly) to fix 9067

Fix #9067 
Fix #9035 
**CC:**@RaymondLim 

The logic to sync the selection marker has been reintroduced and the CSS to draw the triangle was removed.

the scroller shadow is that line at the bottom of the view that you see in #9084 so that won't work.  I've tried adding a class to any view that gets a sidebar selection extension and overriding the scroll thumb for that 

```
.sidebar-selection-extension ::-webkit-scrollbar-thumb { 
    z-index: @z-index-brackets-selection-extension + 1;
}

```

That didn't work.  I tried many different variations, `9999`, adding it to ` :hover::-webkit-scrollbar-thumb:vertical`, etc... I even tried adding to every `::webkit-scrollbar` / `::webkit-scrollbar-thumb` selector in `brackets_scrollbars.less` to no avail.  Either webkit ignores this or doesn't know what to do with it since the element it's attached to still has a lower z-index than the selection extender.