Enabled visual feedback when clicking on the empty symbol gatom to edit its value

* Before empty symbol gatom had no feedback when being clicked on and one had to start typing trusting that the object is indeed properly activated.

* Now, the object draws ... as soon as an empty (and only empty) symbol gatom is selected, so that it has text to highlight thereby letting the user know that the object is ready to receive their input. In case the symbol gatom already has content, then that content gets highlighted which serves the same purpose.

* This does not affect the float gatom.