Add optional constructor arguments to more Control nodes

This can be used to make editor code more compact.
However, as of writing, these constructor arguments cannot be used
from the scripting API.

This was already provided for Label and CheckBox, but it was missing
for other Control nodes where it made sense to provide a default value.

scene/gui/text_edit.h

@@ -940,7 +940,7 @@ public:
 	void set_draw_spaces(bool p_enabled);
 	bool is_drawing_spaces() const;
 
-	TextEdit();
+	TextEdit(const String &p_placeholder = String());
 };
 
 VARIANT_ENUM_CAST(TextEdit::CaretType);