Fix KTimeComboBox for locales with unusual characters in formats

KTimeComboBox creates an edit mask by replacing the AM/PM indicator in a formatted time with 'a' ("optional alphabetic") mask characters. This does not work for locales that use punctuation in the indicator, such as the current en_CA format "9:00 A.M.". This patch uses mask character 'x' ("optional non-blank").

The current fr_CA time format contains an embedded 'h': "9 h 00". 'h' is an edit mask character ("optional hexadecimal digit"), so the generated mask rejects properly-formatted times. This patch escapes all characters that are not supposed to be mask characters.

Credit to Filipus Klutiero for a detailed explanation of the errors.

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