GTextEditor: Fix wrong width calculations with line-wrapping enabled

There were various little mistakes in the width calculations used by
the line-wrapping layout code.

With this patch, we should no longer see the horizontal scrollbar get
enabled with line-wrapping enabled. I will hide the scrollbar in a
separate patch.

Libraries/LibGUI/GTextEditor.h

@@ -199,7 +199,6 @@ private:
         StringView view() const { return { characters(), length() }; }
         const char* characters() const { return m_text.data(); }
         int length() const { return m_text.size() - 1; }
-        int width(const Font&) const;
         void set_text(const StringView&);
         void append(char);
         void prepend(char);