Disambiguate selected vs hovered states for GridDelegate

Right now the GridDelegate component uses nearly the same visual style for both its hovered and selected states (a blue ring drawn around the grid item), with the only difference being a slight lightening effect for the hovered item. This effect is often practically invisible, especially with a light color scheme.

This commit matches the semantics of list items and uses a lighter color for the hovered state compared to the highlighted state.

In addition, the selected item now uses bold text, which is a convention we use in other places too.

BUG: 406914 FIXED-IN: 5.79

Here's how it looks now:

simplescreenrecorder-2021-01-22_09.52.43

cc @teams/vdg