Avoid stacking of multiple licensing warnings in Composer

[RealRaven2000]

Symptom - vairous notficiation stacking up:

in older Thunderbird versions, these used to be stacked behind each other

ideally:

• only show a single notification (prioritize the most important one), the trial period one can definitely be removed when any "feature triggered" ones are shown
• only show the trial message occasionally (every 21 days?)