IDE-1567 Add LiferayMessageErrorTagCompletionProposalComputer for lifera...

IDE-1567 Add LiferayMessageErrorTagCompletionProposalComputer for liferay-message/liferay-error tag
static is bad here, because all keys for all computers (across different files) will be cached which is not what you would want.  The cache should be just per file, and you need to determine how the computers are created, once per file?  once per editor? once per VM session?