Sidebar widget hover, selection highlight and sizing isn't the same as granite.sourcelist

## What Happened

Here are a list of inconsistencies with the new implementation:
1. Hovering over the section expander animates the arrow with a fade-in rather than instantly.
2. Hovering over the section expander doesn't show the same white highlight
3. Section expander doesn't have the same text weight and colour
4. Section expander isn't the same thickness
5. Keyboard navigation and clicking section expanders don't have any styling
6. Sections animate opening and closing (Yes it looks cool, but I don't think it's worth loosing the visual consistency)

## Expected Behavior

The sidebar behaves consistently with other sidebars both visually and when navigating.

## Steps to Reproduce

1. Open files
2. Open an app using sourcelist
3. Compare the behaviour of interacting with them

## Platform Information

elementary OS Daily, both master and @jeremypw 's merged-fixes branch