Restyle GsSummaryTile and use on overview and category pages

This implements the new tile style from

• https://gitlab.gnome.org/Teams/Design/software-mockups/-/blob/master/adaptive/category-page-mvp.png
• https://gitlab.gnome.org/Teams/Design/software-mockups/-/blob/master/adaptive/explore-page-mvp.png

Changes to the containers for the tiles (to get the right number of rows and columns, etc.) will be done as a separate MR.

Signed-off-by: Philip Withnall pwithnall@endlessos.org

Helps: #1111 (closed)