Update BUILDING.md

Correction of a typo and a few style incongruences.

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Closes #1750
Fix emoji backwards compatibility recents crash

The old emoji drawer stored emoji with a .png suffix. Replace it during
list deserialization.
Add a backspace key to the emoji drawer
Remove gray placeholder squares while loading emoji
Occupy all vertical space for emoji grid

Fixes the recent emoji list leaving an empty gap at the bottom of the
drawer depending on the number of items shown.
Whoops, add GcmRegistrationService to Manifest.

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Updated language translations.

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Bumping version to 2.1.2

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