ECMDeprecationSettings: enable warnings by default

The main target consumers of this macro currently (should) want to be informed about new deprecations as early as possible. As do the authors of the warnings.

So instead let's make no warnings an opt-in. Not documenting the now deprecated flag in the docs, given no wide-spread use yet (only found usage with Konsole & Dolphin, but not all the other usages outside frameworks), just supporting still in the code with a note to users. (Update: kdenlive just started to use it, too, still just 3 that need adaption :) ).

@alex @vkrause @ahmadsamir @dfaure @mlaurent