Improve the previewer behaviour.

The previewer seems to have been designed to show certain buttons depending on the type of file to preview: every time a new file is selected it gets repainted or repopulated with icons.

The problem is that it happens on every file change, even if the type is the same as the previous one resulting in unnecessary repainting. Think for example when dealing with text files: switching from one to another should only required updating of the text view, not the whole previewer.