Add wxDocManager::FindDocumentByPath() helper.

Simply refactor the code which already existed inside wxDocManager in a new
public method.

Closes #15126.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73730 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/changes.txt

@@ -625,6 +625,7 @@ All (GUI):
 - Make wxGenericDataViewCtrl::SetFont() really work (Laurent Poujoulat).
 - Remove wxLogWindow::OnFrameCreate(), it was never called anyhow.
 - Added wxDocument::Activate() (troelsk).
+- Added wxDocManager::FindDocumentByPath() (troelsk).
 
 wxGTK: