App title and sub_title should be converted to strings

[willmcgugan]

I'm thinking that App.title and App.sub_title should be converted to strings.

Balancing convenience with hiding errors, this is probably the right thing to do 90% of the time.

[davep]

(More a thought for a spin-off issue than something for this issue) With the same motivation: should we do the same for border titles too?

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