Add recursive browsing options to preferences

Allow maximum recursion depth and maximum sub-directories to be
configured in preferences.

rtgui/preferences.h

@@ -194,6 +194,8 @@ class Preferences final :
     Gtk::CheckButton* overlayedFileNames;
     Gtk::CheckButton* filmStripOverlayedFileNames;
     Gtk::CheckButton* sameThumbSize;
+    Gtk::SpinButton* browseRecursiveDepth;
+    Gtk::SpinButton* browseRecursiveMaxDirs;
 
     Gtk::SpinButton*  threadsSpinBtn;
     Gtk::SpinButton*  clutCacheSizeSB;