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.
XLIFF Parser fails for xliff 1.2

The xliff code should check the first node for various namespaces (not
just 1.1), at least if translate tries to claim 1.2 support (which it
does, according to the TC xliff support survey).

Fixes bug 2948.
Add unit test for the previous commit
Revert "Ignore accelerators check for Bengali and Hindi"

This reverts commit 09df61a270aeb7c523065f4371755e70f050c12b.

We need to associate this with specific project and not ignore the
checks globally.
correct spelling of Finnish (Suomi)
Avoid bogus TypeError exception when raising ValueError

In the else: case here, both inputext and templateext are always empty/None,
so there is no point trying to use them to format ValueError in this code
clearly copy-pasted from above.
Ignore compiled Java properties.class if it is generated for testing
Add basic support for Turkish

 - extra 6 characters for valid accelerators (available on Q and F keyboards)
 - tests for accelerators

Fixes bug 1089
Generate pinned and minimum requirements

For testing and deployment purposes, it is very helpful to have pinned
requirements (that won't change when a new release comes out).  It is
also useful to be able to test the purported minimum versions that are
supported, to see if that is in fact true.

This commit does not set up that testing infrastructure, but does add
minimum versions to many requirements, and generates requirements.txt
and a new min-required.txt with minimum claimed supported versions.

This is part of the resolution for Bug 3005.