Upgrade to LTS 13.3

See merge request decker/decker!61

- and rename module Lens to Text.Pandoc.Lens

Resolve "Wrong keyboard shortcuts in example"

Closes #119

See merge request decker/decker!59

Resolve "Build system on Windows"

Closes #113

See merge request decker/decker!65

- Copy and modify Markdown.hs from Pandoc 2.5
- Copy the Text.Pandoc.Lens module because the repo does not seem
  to be maintained any longer.

Resolve "Use of Font Awesome icons is broken"

Closes #99

See merge request decker/decker!55

Fix URL paths for windows

Closes #115

See merge request decker/decker!58

Modified example to work with the master again and corrected some minor errors…

Closes #55

See merge request decker/decker!53

Resolve "Use Haskell zip to extract the resource files from the executable"

Closes #102

See merge request decker/decker!56

Resolve "Version with preextracted resources"

See merge request decker/decker!57

Include source code from local source files

See merge request decker/decker!54

# Conflicts:
#   test/Spec.hs

Add slide ids and an index file for the Sketch Pad app

Closes #110

See merge request decker/decker!51