Add Language::IsRTL() helper function.
Fixes to editing RTL language files under LTR locales.

Properly align the text control as well as list entries when showing a
PO file that uses RTL (e.g. Arabic) when Poedit's UI language is English
or another LTR language.
Merge branch 'rtl-fixes' into stable
Auto-clear previous-msgid when clearing fuzzy flag.

When the user edits a string with fuzzy flag set and with previous msgid
(#|) entry, clear the previous msgid once the item is edited and the
fuzzy flag cleared.

Without this, Poedit would always keep the previous-msgid and would save
it even for non-fuzzy entries.  This was wrong.