widgets.main: use "Staging Area" for better i18n

Closes #283

Suggested-by: Ｖ字龍(Vdragon) <pika1021@gmail.com>
Signed-off-by: David Aguilar <davvid@gmail.com>
git-cola.rst: use "Diff Region" for consistency

Signed-off-by: David Aguilar <davvid@gmail.com>
doc: update v2.0.4 release notes draft

Signed-off-by: David Aguilar <davvid@gmail.com>
po: update i18n for "Diff Region" change

This does not actually update the translations.  Replace the term
"Section" with "Diff Region" since the spirit of the phrase should be
the same in the current translations.  This does not touch zh_TW since
Vdragon will probably update it.

Signed-off-by: David Aguilar <davvid@gmail.com>
po: update git-cola.pot

Signed-off-by: David Aguilar <davvid@gmail.com>