New placeholder property and various caption rendering enhancements

[kartik-v]

• New placeholder property via new msgPlaceholder property. Defaults to Select {files}... where {files} will be replaced with filePlural string for multiple uploads and fileSingle string for single file uploads. To not display a placeholder set msgPlaceholder to null or false.
• Caption container and caption name templates are now enhanced to use a native text input to better work with bootstrap input groups
• Enhance rendering of the file caption icon
• Enhance error display and caption error styling for bootstrap v4.x