Allow building multiple language packs in one run

Fixes bug 2999
Remove the check that source and target are the same

Fixes bug 2805.
Add unit test for the previous commit
Add --removeuntranslated option to po2dtd and po2moz

Fixes bug 1718
Add --threshold option to po2* converters

An option to specify the percent of translated words below it no file is
output.

Fixes bug 2998
Add -S, --timestamp option

To skip conversion if the output file has newer timestamp
(Makefile-like).

Fixes bug 3010
Fix tests
Revert "Fix tests"

This reverts commit b2058d7b05980d67cfa3979a89eea2f4a0f6ae1e.
Properly fix the tests

It is a common ConvertOptionParser option, so should be tested in the
base TestConvertCommand class.
Lower the severity of the accelerators check

I was not sure what severity to use, so lowered it to "functional" but
lowered it further to "cosmetic" in Mozilla checker, but may be we are
better using just one of them.

Related to bug 2708.
Ignore accelerators check for Bengali and Hindi

Just an example for bug 2708, we need a complete list of languages where
this is desirable.