Add userDismissible option for toasts

[EliasBinder]

Proposed changes

Introduced an additional option for toasts to allow them only to be dismissed programmatically and not by swiping. However, this option defaults to true, so there are no behaviour changes regarding the current version in case this option is omitted in M.toast({...}).

This can be useful for several cases:

• when a preloader bar is included in a toast to show the progress of an async file upload.
• when the user is offline. The toast can then be dismissed as soon as connectivity is restored.
  ...

Screenshots (if appropriate) or codepen:

Types of changes

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to change).

Checklist:

• I have read the CONTRIBUTING document.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.