New feature + bug fix

Using H, you can scroll to a folder in the foldertree without changing the messagelist. This helps to see folders 'near' the chosen (to see an exact foldername you don't remember), or you can drag and drop messages from the messagelist to the folder now displayed in the foldertree.

The number of folders being displayed in the folderlist can be set to your preferred value.

The keys are now (again) active in all tabs, because recognising tab type was not totally reliable. If this interferes with your use (that was once reported for calendar entries), then you can revert this change in the options (restrict commands to email tabs).