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/circle/circletool.cpp

@@ -39,7 +39,7 @@ QString CircleTool::nameID() {
 }
 
 QString CircleTool::description() const {
-    return tr("Sets the Circle as the paint tool");
+    return tr("Set the Circle as the paint tool");
 }
 
 CaptureTool* CircleTool::copy(QObject *parent) {