NLS: add Multi_key sequences for double grave and inverted breve

jmcwilliams403 requested to merge (removed):doublegrave-invertedbreve into master

Most keyboard layouts don't have the dead keys required to type these symbols. This makes them typable with Multi_key. The <grave> <grave> prefix comes from the existing sequences for Cyrillic NFD's towards the bottom of the file, while the <parenright> prefix comes from the mirror of the <parenleft> postfix used by Ăă and Ğğ for normal breve.

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