Brief summary of issue / Description of requested feature:

Mudlet has an option to alert on the taskbar if new text is received. However, the option doesn't take into account if the recieved text is deleted by triggers and nothing ends up being displayed: in this case Mudlet still alerts even though there is nothing to see.

The alert feature should take triggers deleting the text entirely into account.

Steps to reproduce the issue / Reasons for adding feature:

1. connect to a game and
   import delete all incoming lines.zip
2. open the tab for a 2nd game
3. go back to the first game, reconnect using Alt+R, and quickly switch tabs to the 2nd game
4. observe how the 1st game tab is being highlighted even though no new text should be on the screen:

Error output / Expected result of feature

No notification is shown if all incoming text has been deleted and nothing ends up being shown.

Extra information, such as Mudlet version, operating system and ideas for how to solve / implement: