Remove previous and next buttons in displayimage if not required.

[Nibbler]

If the picture is the first in an album do not display the back icon.
If the picture is the last in an album do not display the next icon.

See http://forum.coppermine-gallery.net/index.php?topic=45485.0 but do it properly. 

[Paver]

For 1.5, I think it's better to implement this in themes.inc.php and each theme.php in $template_img_navbar and function theme_html_img_nav_menu.

I thought about using a blank image but then decided it looked better in my eyes to have an inactive-looking image instead (as a placeholder for what's there for other images).  I created a black & white version of next.png and prev.png since that was simplest to create next_inactive.png and prev_inactive.png (all in images/navbar/).  These could be replaced with blank images or better disabled-looking images if desired.

There was a $lang_text_dir check & tag that was not used so I implemented by how I assumed it was meant to be.  I tried to work through in my head how a right-to-left reader would want to see the nav buttons and links, but I couldn't figure it out, to my embarrassment.  The non-implemented code looked like the intent was to merely swap the images so that's what I did.  If that's wrong, we'll have to fix it.

Committed the inactive images and code to 1.5 for themes.inc.php and sample/theme.php.

Need to create inactive images and add code for the rest of the themes in 1.5.