check for errorstring of the correct variable

each protocol needed to create the editor on their own.
Instead emit fetchDone and let the plugin instantiate it for
every protocol

- created protocol class as basis for different paste servers
- removed custom classes and replaced functionality with
  simple QHttp* usage
- removed poster and fetcher classes copied from cpaster
  application. It not getting updated anyways in creator
- Known issue: Listing does not update, when user changes
  protocol
- TODO: add pastebin.ca support. Code is done already, just
  needs to be placed inside plugin.