Themes can set minimum thumbnail size when calculating proportions.

This prevents broken item rendering when thumbnails are smaller than the
theme design allows.

Trac-Ticket: #1838
Themes can choose child thumbnail when asking for proportion.

This helps themes better calculate an item's container size, for cases
when the first item significantly differs in size from the rest.

Trac-Ticket: #1838
Merge pull request #204 from jozefs/issue1838

#1838 - thumb_proportion should have a minimum size.