Make tools' (English) tooltip phrasing consistent (#375)

Some tooltips were written as imperatives ("Set foo to bar"), others
were written as simple present ("Performs foo action"). All should now
be consistently written in imperative form (unless I missed any…).

src/tools/copy/copytool.cpp

@@ -40,7 +40,7 @@ QString CopyTool::nameID() {
 }
 
 QString CopyTool::description() const {
-    return tr("Copies the selection into the clipboard");
+    return tr("Copy the selection into the clipboard");
 }
 
 CaptureTool* CopyTool::copy(QObject *parent) {