Change the way the plugin checks for valid filenames

The plugin now checks if a given file name is valid in the follwing steps:
 - Check if a file with such a name exists; ask for override confirmation
   if that's the case
 - Try to open the given path to check if it's valid
	- If it's invalid ask for a new filename
   	- Create the snippet otherwise

This also has the sideeffect that it's possible to create a snippet in a
subdirectory instead of directly in the User folder.