thumbnail: decouple proc params rank/color/trash

Add a Properties type that contains values for rank, color and trashed
Each value will also contain an edited flag.

The properties variable is used to store rank, color and trashed. They
are fetched from the proc params and will update the proc params when
needed.

This is the base for future implementations where some properties (like
rank) will be also retrieved from other sources like xmp sidecar files.

rtgui/thumbnail.h

@@ -78,6 +78,30 @@ class Thumbnail
 
     bool            initial_;
 
+    // Properties holds values and edited states for rank, color and trashed
+    struct Properties {
+        template <class T> struct Property {
+            T value;
+            bool edited;
+            Property(T v): value(v), edited(false) {}
+            Property& operator=(T v)
+            {
+                value = v;
+                edited = true;
+                return *this;
+            }
+            operator T() const { return value; }
+        };
+        Property<int> rank;
+        Property<int> color;
+        Property<bool> trashed;
+
+        explicit Properties(int r=0, int c=0, bool t=false):
+            rank(r), color(c), trashed(t) {}
+        bool edited() const { return rank.edited || color.edited || trashed.edited; }
+    };
+    Properties properties;
+
     // vector of listeners
     std::vector<ThumbnailListener*> listeners;
 
@@ -90,6 +114,8 @@ class Thumbnail
     Glib::ustring    getCacheFileName (const Glib::ustring& subdir, const Glib::ustring& fext) const;
 
     void saveMetadata();
+    void loadProperties();
+    void updateProcParamsProperties();
 
 public:
     Thumbnail (CacheManager* cm, const Glib::ustring& fname, CacheImageData* cf);