Add recursive sub-folder image browsing

Add toggle button in the top toolbar of the file browser to activate or
deactivate recursive browsing. Toggle state is saved in options.

Limit recursion depth and explored directory count.

rtgui/options.h

@@ -315,6 +315,9 @@ public:
     std::vector<int> parseExtensionsEnabled;      // List of bool to retain extension or not
     std::vector<Glib::ustring> parsedExtensions;  // List containing all retained extensions (lowercase)
     std::set<std::string> parsedExtensionsSet;  // Set containing all retained extensions (lowercase)
+    bool browseRecursive;
+    int browseRecursiveDepth;
+    int browseRecursiveMaxDirs;
     std::vector<int> tpOpen;
     bool autoSaveTpOpen;
     //std::vector<int> crvOpen;