PPM: fixed non-functional mouse cursor after resizing columns

Here's the report on the bug:
http://www.murga-linux.com/puppy/viewtopic.php?t=91038

Side effects of my modification are:
- it's necessary to double-click a package name in order to (un)install it, although for categories single-click is still valid
- it's possible to choose a package also using a keyboard (space/enter), not only a mouse

The 'single->double click' change might be considered as "radical", but I'm pretty sure that it won't be confusing for more than few seconds - if single-click doesn't work, the natural human reaction (developed by millenia of evolution) is to try double-click.

Greetings!