Add property to control display of the personPhoto placeholder

The personPhoto.png image is displayed in the call panel of audio calls if we have no photo of the other party.

This change creates a new option allowing the image to be hidden (and the window size reduced) if there is no photo of the other party.

```
net.java.sip.communicator.impl.gui.main.call.HIDE_PLACEHOLDER_PIC=true
```