Show a checkbox in the corner for checkable cards
Cards can be checkable. However, when they're checkable, nothing visually suggests this, so it's not clear to the user.
To fix this, we can add a little checkbox to the corner of checkable cards. To make that happen, we have to add the functionality to BannerImage, since that's what draws the header. In BannerImage, it's an optional and off-by-default thing, and Card opts into it.
There is no visual change for non-checkable cards.
Here's how it looks in the portal screen chooser dialog (which has some checkable cards) in conjunction with plasma/xdg-desktop-portal-kde!263 (merged):
Edited by Nate Graham