LocatorFilterEntry: Add linkForEditor

Make it of std::optional<Utils::Link> type.

Use it for:
1. Removing duplicates
2. Showing link inside editor

Before, QVariant internalData was used in above cases.

Rationale:
1. Removing duplicates between results from different
   ILocatorFilter makes only sense if accept() overload
   for these filter types are the same (i.e. the implementation
   is repeated). Otherwise, we are loosing some functionality
   when some result is removed. Taking into account that
   original intention was to eliminate duplicates for filters
   where we do BaseFileFilter::openEditorAt() inside accept(),
   we store linkForEditor in 3 cases (instead of storing
   internalData):
 - BaseFileFilter
 - FileSystemFilter (only existing file case)
 - OpenDocumentsFilter

   We don't save a linkForEditor where we stored QString
   inside internalData in the following cases:
 - JavaScriptFilter
 - FileSystemFilter (non existing file case)

   Before, in above cases, we could have filtered out
   some results unintentionally. This patch should fix it.

   Now, we remove duplicates only for enties that have value
   for linkForEditor.

2. Use directly linkForEditor, if exists, for opening editor.

This patch aims to decrease the number of different
responsibilities of internalData by 1.

Make 3rd arg of LocatorFilterEntry c'tor default.

Change-Id: I144c58661d7456bb5991f8077dc103272dfab194
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/macros/macrolocatorfilter.cpp

@@ -50,7 +50,7 @@ QList<Core::LocatorFilterEntry> MacroLocatorFilter::matchesFor(QFutureInterface<
         }
 
         if (index >= 0) {
-            Core::LocatorFilterEntry filterEntry(this, displayName, QVariant(), m_icon);
+            Core::LocatorFilterEntry filterEntry(this, displayName, {}, m_icon);
             filterEntry.extraInfo = description;
             filterEntry.highlightInfo = Core::LocatorFilterEntry::HighlightInfo(index, entry.length(), hDataType);