Omittable: split out from util.h and refresh

Improvements:
- Quotient::lift() - a way to invoke a function on an optional (including
  Omittable) or a pointer if it's 'truthy'. Doesn't need enhanced
  function_traits<>, only the standard library; works on any number
  of arguments that can be dereferenced and casted to bool.
- then() - the version of lift() as a member function.
- edit() was renamed to ensure() (edit() might become a read-write
  counterpart of then() at some point). It's not really used across
  libQuotient codebase (or elsewhere) but is staying there just in case.
  It can also accept an initializer, removing the requirement of
  default-constructibility.
- Quotient::merge() is simplified, with one universal implementation
  covering both Omittable/optional and plain values.
- All that now lives in its dedicated pair of files, further
  decluttering util.h

1 files changed, 1 insertions, 0 deletions

diff --git a/lib/converters.h b/lib/converters.h
index 8eea1cd3..e7bc6898 100644
--- a/lib/converters.h
+++ b/lib/converters.h
@@ -3,6 +3,7 @@
 
 #pragma once
 
+#include "omittable.h"
 #include "util.h"
 
 #include <QtCore/QDate>