Consolidate input completion code in preparation for advanced completion strategies (#4639)

Co-authored-by: Rasmus Karlsson <rasmus.karlsson@pajlada.com>

src/common/ChatterSet.hpp

@@ -39,6 +39,10 @@ public:
     /// are in mixed case if available.
     std::vector<QString> filterByPrefix(const QString &prefix) const;
 
+    /// Get all recent chatters. The first pair element contains the username
+    /// in lowercase, while the second pair element is the original case.
+    std::vector<std::pair<QString, QString>> all() const;
+
 private:
     // user name in lower case -> user name in normal case
     cache::lru_cache<QString, QString> items;