PixelPaint: Add Tool::tool_name() as a single-point-of-truth

Let the tools know what their names are.

Userland/Applications/PixelPaint/Tools/Tool.h

@@ -74,6 +74,8 @@ public:
     GUI::Action* action() { return m_action; }
     void set_action(GUI::Action*);
 
+    virtual StringView tool_name() const = 0;
+
 protected:
     Tool() = default;
     WeakPtr<ImageEditor> m_editor;