Add exercise 6: Use target files

- Motivate from portablility of sources ("allow other developers to
  work on the project")
- Re-use target file once available.
Make recovery option more explicit
Fix screenshot
Fix target platform configuration

- Use fixed GAVs
- tpconfig is not there yet (if no bonus done)
Use latest tycho version 0.14.1
Update exercise 2 to e4-based RCP
Remove exercise 6 and solution
Add new solution of exercise 5 (e4-based)
Update exercise 1 to use e4 RCP
Constructed starting point for exercise 4 from exercise 5 solution

- URLs of local p2 repositories are not yet consistent; using
  "file:/PATH/TO/tutorial/p2_repository/" as in exercise 1 readme
Automatic formatting of exercises 4 and 5 readme.html
Instructions for exercise 4
Update exercise 2 README
Update starting point for exercise 3
Update README for exercise 3
Exercise 2: Add forgotten build success screenshot
Exc 4: Add change p2 URL step to recovery option
Exc 5 solution: Use file:/ p2 URL
Starting point for exercise 5

- Constructed from executing exercise 4
Instructions for exercise 5