Change the way the plugin checks for valid filenames

[sascha-wolf]

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.