Should be able to cancel while it is working

[AlisterH]

If someone accidentally tries to use an input file for example with a very large number of features, it can take a very long time to generate a coverage layer (and use a lot of ram), and maybe it could freeze QGIS. A cancel button would be a good idea!

#8 would likely solve this.

[AlisterH]

Also, we could:

• provide advice/warning to users on the number of features
• check the number of features isn't too many
• check if the user wants to continue if the task takes longer than a certain amount of time
• dissolve features to reduce the number
• look into refactoring our code to make it more efficient, or using some other exiting tool
  I'm not sure that simplifying features helps at all.

I believe this issue is the only real reason for keeping the plugin marked as "experimental".