New file box now has default file name + extension

Whenever a user creates a file or folder in the web UI, the input field
will contain a default file name, pre-selected up to the extension for
easier typing.

The purpose is mostly to prevent users creating text files without an
extension.

Fixes #6045

Here is a screenshot of the action:
![newtextfile](https://f.cloud.github.com/assets/277525/1638560/481fccc0-580f-11e3-87eb-8d8e255d8dbc.png)

Tested in Chrome, Firefox and IE 8. If the file "New text file.txt" already exists, the error isn't shown directly but only after hitting enter, which is acceptable. (showing the red directly would have a "shocking" effect)

Please review @jancborchardt @karlitschek @DeepDiver1975 @schiesbn @kabum