Implement DocIterator::backwardInset()

I thought I would need it to fix bug #9418, but once backwardInset() worked
it turned out that it is not needed. However, since it took me some time to
figure out the correct implementation I do not want to throw the result away.

src/DocIterator.cpp

@@ -449,6 +449,28 @@ void DocIterator::backwardPos()
 }
 
 
+#if 0
+// works, but currently not needed
+void DocIterator::backwardInset()
+{
+	backwardPos();
+
+	while (!empty() && !nextInset()) {
+		if (inTexted()) {
+			pos_type const lastp = lastpos();
+			Paragraph const & par = paragraph();
+			pos_type & pos = top().pos();
+			while (pos > 0 && (pos == lastp || !par.isInset(pos)))
+				--pos;
+			if (pos > 0)
+				break;
+		}
+		backwardPos();
+	}
+}
+#endif
+
+
 bool DocIterator::hasPart(DocIterator const & it) const
 {
 	// it can't be a part if it is larger

src/DocIterator.h

@@ -202,7 +202,7 @@ public:
 	/// move backward one paragraph
 	void backwardPar();
 	/// move backward one inset
-	/// FIXME: This is not implemented!
+	/// not used currently, uncomment if you need it
 	//void backwardInset();
 
 	/// are we some 'extension' (i.e. deeper nested) of the given iterator