Use elliptic curve as a default implicit curve

The current default implicit plot 'y∙sinx + x∙cosy = 1' scales badly on the big area. This can be mitigated using some simpler but famous plot.

BUG: 201515
Differential Revision: https://phabricator.kde.org/D21190

kmplot/functioneditor.cpp

@@ -511,7 +511,7 @@ void FunctionEditor::createImplicit()
 	if ( Settings::defaultEquationForm() == Settings::EnumDefaultEquationForm::Function )
 		name += "(x,y)";
 	
-	createFunction( name + " = y*sinx + x*cosy = 1", QString(), Function::Implicit );
+	createFunction( name + " = y² = x³ − x + 1", QString(), Function::Implicit );
 }