Deselect declarations transitively depending on omitted declarations

[dschrempf]

Warn user in such cases.

Based on #1314 which should be merged first.

Travis and I agreed that trace messages should start with captial letters.

[dschrempf]

Add another level here.

[dschrempf]

Test 1: Prescriptive binding with "omit foo", but we use foo (directly or
transitively). Check for correct/expected message.

Test 2: External binding specification with "omit foo".

• Header 1: Does define foo and matches against external binding specification.
• Header 2a: Does not use foo -> No messages at all.
• Header 2a: Uses foo -> We generate bindings for the declaration foo that
  was omitted by an external binding specification. At the moment don't issue a
  warning but create a ticket: We could emit a warning in this case.