QStaticText: mark constructor as explicit

A QStaticText object is supposed to be created and *kept* by the user,
and then used repeatedly for drawing, saving the expensive
layouting/rasterization process which would otherwise be done for each
drawText-like call.

Allowing an implicit conversion from QString is therefore surely
unwanted.

[ChangeLog][Potentially Source-Incompatible Changes][QStaticText] The
QStaticText(const QString &) constructor is now explicit.

Change-Id: Ib939a5f46638cb9a54d1cc30ff50e3a9d20be5e4
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

src/gui/text/qstatictext.h

@@ -62,7 +62,7 @@ public:
     };
 
     QStaticText();
-    QStaticText(const QString &text);
+    explicit QStaticText(const QString &text);
     QStaticText(const QStaticText &other);
 #ifdef Q_COMPILER_RVALUE_REFS
     QStaticText &operator=(QStaticText &&other) Q_DECL_NOTHROW { swap(other); return *this; }