Filter Action label does not handles names containing a slash

[ThHareau]

When choosing the label action while creating a filter, labels containing a / are not handled correctly.

To reproduce:

1. Create a filter

   • Create a label that contains a / (example: la/bel);
   • Create a new filter;
   • Choose a valid condition;
   • Choose the label with a /
   • Save.

2. Open the related sieve script

   • Notice that the label slash is not escaped;
   • Escape the slash, buy appending \\ before it. If the label was la/bel, it should now be transformed to la\\/bel. That is how the label should be given to be understood by the sieve library;
   • Save

3. Edit the filter using the graphical tool

   • Notice that the label is no longer present in the label action