Do not block UI thread on database requests

[paetz]

When writing to database, we are currently blocking the main thread. The better approach is to move this to a separate coroutine and display an hourglass in the UI (to still block user interaction).
The following actions should be considered:

• Import Tourbook
• Export Tourbook ??
• Write elements to db during sector update
• Reset db
• Deletion of regions in the RegionManager

[chrgernoe]

We should definitely do this also for the tourbook export. Especially, if the tourbook is huge the app hangs a while.