Autocomplete placeables in translation text area

Add support for placeables autocompletion in the translation text area, plus small fix to the appearance of the placeables in RTL text.

For testing:
http://textarea-autocomplete.testing.locamotion.org/

e.g.
http://textarea-autocomplete.testing.locamotion.org/am/pootle/translate/pootle.po#unit=36372