Fix issues #128, #126, #91, #21



#128 - file browser now only shows valid file types by default
#126 - a 'no player found' message is displayed when no players are found for the current search string
#91 - all changes are reverted when pressing the 'cancel' button, including prop unassignment and user access level changes. Prop viewing and unassigning on the stage edit page has been improved. It is currently not possible to preview audio or streaming avatars on the stage edit page; this will be added to the issues log as a future enhancement. Currently one bug in this feature - if the user makes changes and then navigates away from the specific stage edit page, the changes are neither saved nor cancelled but stored in their temporary state and are still there when the page is reloaded. This will be added to the issues list as a bug.
#21 - was fixed incidentally while working on #91