Update wip/vimb to vimb-2.6.

Changes (from vimb-users@ ML):
Features:
- allow to use external tool or script to handle downloads
  This can be enabled with the new setting 'download-use-external'. The tool
  can be set with ':set download-command'. By default vimb still runs download
  through webkit built in mechanism.
- fire ambiguous numeric hints after an timeout
  If hints are filtered by number, the first (current highlighted) hint is
  fired automatically after a short timeout. The timeout can be changed during
  runtime by ':set hint-timeout', default value is 1000ms.
- don't allow to quit vimb if webkit is still running a download
  To force quit vimb the bang was introduced to quit command ':quit!'.

Fixes:
- removed memory leak in handler processing
- simplified the way the runtime settings are processed in vimb
- use g_slice_new instead of g_new* to follow the recommendations of the glib
  team
- added missed quote in some default shell commands used by vimb