Add --add-comments=TRANSLATORS to xgettext call by default.

This helps with the common case of prefixing comments with TRANSLATORS;
if that's not suitable, people can always change it.
Better detection of DOS absolute paths in FileViewer.

The code got confused by paths with incorrect path separator, such as
"c:/Projekty/.../foo.java:86.  It's a weird case, but making it work was
cheap, so why not.

Fixes #535.
Update Lithuanian translation.
Fix typo in German translation.
More updates to Kirghiz translation.
Always focus the Find text field.
Show UTF-* encoded files correctly in the file viewer.

Correctly recognize UTF-8/16 encoded sources files when displaying them
in wxSTC.

Fixes #521.
Try harder when finding source files, look in parent dirs too.

Sometimes, the path in source reference is not relative to the PO file's
location, but is relative to e.g. the root directory. See
https://code.djangoproject.com/ticket/13936 for exhaustive discussion
with plenty of examples.

Deal with this by trying parent directories of m_basePath too. So if a
file named project/locales/cs/foo.po has a reference to src/main.c, try
not only project/locales/cs/src/main.c, but also
project/locales/src/main.c and project/src/main.c etc
Increase version to 1.5.6, update NEWS.