fix #294121: navigation skips annotations entered out of order

Annotations are appended to the annotation list as they are entered.
This means they won't necessarily be sorted by track,
if you enter them onto staves in any order but top down.
The result is the navitgation code skips all annotations for a staff
after the first annotation it encounters on a different staff.

This commit fixes the issue by continuing to loop through the annotations,
looking for more on the same staff.

anatoly-os: rewrite the improvement from #5308 using `find_if`

anatoly-os: Did refactoring using `find_if` to make the code cleaner. Initial idea and implementation are authored by @MarcSabatella.

libmscore/segment.cpp

@@ -1178,11 +1178,15 @@ Element* Segment::nextAnnotation(Element* e)
       {
       if (_annotations.empty() || e == _annotations.back())
             return nullptr;
-      auto i = std::find(_annotations.begin(), _annotations.end(), e);
-      Element* next = *(i+1);
-      if (next && next->staffIdx() == e->staffIdx())
-            return next;
-      return nullptr;
+      auto ei = std::find(_annotations.begin(), _annotations.end(), e);
+      if (ei == _annotations.end())
+            return nullptr;               // element not found
+
+      auto resIt = std::find_if(ei + 1, _annotations.end(), [e](Element* nextElem){
+            return nextElem && nextElem->staffIdx() == e->staffIdx();
+            });
+
+      return _annotations.end() == resIt ? nullptr : *resIt;
       }
 
 //---------------------------------------------------------
@@ -1194,11 +1198,15 @@ Element* Segment::prevAnnotation(Element* e)
       {
       if (e == _annotations.front())
           return nullptr;
-      auto i = std::find(_annotations.begin(), _annotations.end(), e);
-            Element* prev = *(i-1);
-      if (prev && prev->staffIdx() == e->staffIdx())
-            return prev;
-      return nullptr;
+      auto reverseIt = std::find(_annotations.rbegin(), _annotations.rend(), e);
+      if (reverseIt == _annotations.rend())
+            return nullptr;               // element not found
+
+      auto resIt = std::find_if(reverseIt + 1, _annotations.rend(), [e](Element* prevElem){
+            return prevElem && prevElem->staffIdx() == e->staffIdx();
+            });
+
+      return _annotations.rend() == resIt ? nullptr : *resIt;
       }
 
 //---------------------------------------------------------