Add filetype sets

[cliffparnitzky]

Add an area to the backend where sets filetypes could be managed.

e.g. Word Docs (.doc, *.docx), Excel files (.xls, .xlsx), Images (.gif, .jpg, .png)

The definied filetypesets should be offered in categories view, as optional checkbox selection (to avoid continuous entering one and the same string values).

The allowed filetypes of a category should be the collection of the defined filetypes and the selected ones from the sets.

[cliffparnitzky]

modify file types:

fields:

1. custom file types (textfield, the actual known one)
2. file type (check box seletion)
3. inherit from categories above

[cliffparnitzky]

ToDo's

• [x] create backend view
• [x] file type toLowerCase (in DMS core too) ... add hint to field, that input will automatically be lower cased and alphabetically sorted
• [x] add project at github
• [x] add to categories view
• [x] implement modificator class

[cliffparnitzky]

Added file type sets into categires view, see ContaoDMS/dms-FileTypeSets@449a38362436437f13e310d29d684f29338d8b71