Exercise Editor: "Enter" should move to next EditText

[ironjan]

Describe the feature you'd like
Hello,
when using the app for the first time, I noticed some peculiarities in the exercise editor. This is applicable to both "free workout" and the program exercise editor.

• Using the Enter-Button skips fields and does not submit
  1. Go to free work out.
  2. Enter an exercise name. Press enter. Focus is on "Sets".
  3. Enter a number. Press enter.
  4. Focus is on "Rest time". Reps and Weight are skipped.
  5. Enter a rest time. Press enter. Nothing happens; I expected that the "Add"-action would be invoked.

Describe alternatives you've considered
Originally part of #147.