LibGUI+LibGfx: Collapse the '&' from Alt shortcuts in tooltip texts

Also resolve a FIXME about using GUI::Label auto-sizing since we're
changing this code and it simplifies what we're doing.

Fixes #6219.

Userland/Libraries/LibGfx/Painter.h

@@ -171,4 +171,6 @@ private:
     Painter& m_painter;
 };
 
+String parse_ampersand_string(const StringView&, Optional<size_t>* underline_offset = nullptr);
+
 }