Bug 1026099 - Rework the spellchecker context menu to not use CPOWs. r=ehsan/billm/felipe

This patch fixes a few nits and typos in the C++ spellchecking code as well as
fixing the Finnish dictionary and multiple installed dictionaries in general (r=ehsan).

It reworks the spellchecking context menu to use async message passing (in JS)
and reduces, by a little, the CPOW use in the context menu code (r=felipe).

Finally, the spellcheck IPDL no longer needs to be 'rpc' since we no longer
nest spellchecking calls, so it can go back to being 'sync' (r=billm).