added new, 3-state setup option to allow skipping levels (yes/no/ask)

added new, 3-state setup option to allow skipping levels (yes/no/ask)

This new setup option replaces the two previous setup options "Force
Solving Levels: yes/no" and "Allow Skipping Levels: yes/no" with a new
option "Allow Skipping Levels: yes/no/ask". This should simplify the
three (useful) combinations of the two previous setup options, as
follows:

- "Force Solving Levels: yes" and "Allow Skipping Levels: yes"
  => "Allow Skipping Levels: ask"

- "Force Solving Levels: yes" and "Allow Skipping Levels: no"
  => "Allow Skipping Levels: no"

- "Force Solving Levels: no" (and "Allow Skipping Levels: yes/no")
  => "Allow Skipping Levels: yes"

This hopefully makes configuring if (and how) unsolved levels may be
skipped or not easier to understand.