Improve file select feedback

Currently if no file or more than 1 file is selected the error "Please select a file" is given. This is confusing especially if more than 1 file is selected.

This is done on line 526 of imce.js

Why not replace line 526 and 527 with:

    if (imce.validateSelCount(1,1)) {
      imce.send(imce.vars.prvfid);
    }

This utilises the neat validateSelCount function already available and improves feedback given to users about their actions. It also lays the foundation for a possible patch to be able to insert more than 1 image at a time from the IMCE File Browser.

I apologize for the way I suggest a code change, I'm not familiar with creating patches yet.