use std::move() instead of copying when value is no longer used

This is mainly to shut down coverity which nowadays nags on this.
I am not sure whether we really gain something in these cases.

src/BufferView.cpp

@@ -1707,7 +1707,7 @@ void BufferView::dispatch(FuncRequest const & cmd, DispatchResult & dr)
 			string label = dit.innerParagraph().getLabelForXRef();
 			if (!label.empty()) {
 				// if the paragraph has a label, we refer to this
-				string const arg = (type.empty()) ? label : label + " " + type;
+				string const arg = (type.empty()) ? move(label) : move(label) + " " + type;
 				lyx::dispatch(FuncRequest(LFUN_REFERENCE_INSERT, arg));
 				break;
 			} else {

src/Encoding.cpp

@@ -798,7 +798,7 @@ docstring Encodings::convertLaTeXCommands(docstring const & str, bool const lite
 		if (!cnvtd.empty()) {
 			// it did, so we'll take that bit and proceed with what's left
 			ret += cnvtd;
-			val = rem;
+			val = move(rem);
 			continue;
 		}
 		// it's a command of some sort

src/insets/InsetNomencl.cpp

@@ -440,7 +440,7 @@ docstring nomenclWidest(Buffer const & buffer)
 				msymb.size();
 			if (wx > w) {
 				w = wx;
-				symb = symbol;
+				symb = move(symbol);
 			}
 		}
 	}