switch to alternative method for generating IDs of new files

[bwbohl]

currently the the value of the form filed filename/ID will be used for:

• FILENAME.xml
• /mei/@xml:id
• /mei/fileDesc/titleStmt/title/text()

At least concerning /mei/@xml:id and the FILENAME this is highly unstable and might lead to invalid filenames or XML data. There are several options, how this could be handled:

1. use uuid generation
2. process field value to a XML conform ID

This of course might be connected to general ID-policies in a specific project and some follow up questions:

• Should ID and filename be identical?
• Should the title in fileDesc be identical to the actual filename?
• …

@nikobeer any preferences?