Fixup 91d1ad31: beware of the reurn values of getPosNearX

The pair returned by TextMetrics;;getPosNearX is (position, boundary).
Here one had to use short variable names p and b because 'pos' and
'boundary' are a members of Cursor, and [b, p] was use dinstead of
[p,b].

Here, we added a bool & version of boundary() instead to the usual
getter/setter paradigm, because pos() is already like that. It might
be that the setter should be removed later.

Using these two members, it is now possible to use std::tie instead of
auto [,].

src/Cursor.cpp

@@ -2286,9 +2286,7 @@ bool Cursor::upDownInText(bool up)
 		}
 
 		Row const & real_next_row = tm.parMetrics(pit()).rows()[next_row];
-		auto [b, p] = tm.getPosNearX(real_next_row, xo);
-		pos() = p;
-		boundary(b);
+		tie(pos(), boundary()) = tm.getPosNearX(real_next_row, xo);
 		// When selection==false, this is done by TextMetrics::editXY
 		setCurrentFont();
 

src/DocIterator.h

@@ -129,6 +129,8 @@ public:
 	///
 	bool boundary() const { return boundary_; }
 	///
+	bool & boundary() { return boundary_; }
+	///
 	void boundary(bool b) { boundary_ = b; }
 
 	// the two methods below have been inlined out because of