#1223 Prevent media file name collisions

[Dave Hieber]

Media file names REALLY need to be determined internally to prevent name collisions. Simply add a uid field to the media table and use a counter to uniquely name the physical files on the server their unique number, or append an easily strippable sequence including the number to the file name, such as my-photo.jpg -> my-photo_1.jpg. Store the orig. file name as you do now for display or batch exporting later. Software as complete as this shouldn't make the user come up with unique names for what could be thousands of items. You should be able to have 5 "picture of dad" images. In a collaborative tool like this, the software needs to prevent accidents like clobbering a file.