Implementation for WICKET-4861.

Added the possibility to select multiple files in the OS dialog for
browsers that support HTML5 'multiple' attribute. For browsers that do
not support it, or if you do not set the boolean to true, it will
behave the same as before.

 New: 
 _ easier select multiple files from multiple folders 
 _ shows the multiple files after choosing them - one per line
 Fixes:
 _ the delete buttons are aligned instead of positioning after the 
 variable length file names 
 _ useless and confusing "C:\fakepath" in filenames reported by some 
 browsers (in multiple with single select mode) are now hidden.