Show or Hide Hidden Files

I attached a commit to enabled possibility to switch show or hide 'hidden' files.

It is configure able by using patterns. I have added a default as example. We can remove this.
```php
$GLOBALS['TL_CONFIG']['hiddenFiles']	      = array('/^\./');
```
The Hidden files are not excluded from sync. You can show them and edit them.
Also it is not possible to select them in the File Picker in an Article -> Do we need that?

You are able to show or Hide hidden Files by clicking on the `showHidden` link in the filemanager.

I know, that patterns are not for the best performance, but IMO it is the best solution.

This Pull Request refers to: #5699 and #5700

So there were discussions about adding a custom file in the Folder similar to the `.gitignore`. @LeoUnglaub and @s-nunez-gosub think that this is a good solution.

So wich way do you think is the best?