dialogs: Use a smaller font-size if the title width exceeds the space

Since quite a few strings of dialogs provided by external programs are not updated yet and the string freeze is already in effect, make sure we don't break those dialogs by stripping aways large parts of the headline.

To do that, detect if the title label is larger than the available width and if it is, switch to a smaller font-size of 13pt. This makes sure we still show about the same number of characters in the headline as we did in previous releases.