Add a garbage collector for note files

Now that we use hashes to identify notes, a garbage collector comes in
handy. We search for note files that aren't referenced anywhere. Such
files might come up if a note, that isn't connected with any other item,
is edited.

Huge parts of this code are very hackish due to our data structure
layout and the way our hash table implementation works.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/calcurse.h

@@ -145,6 +145,9 @@
 #define KEYS_LABELEN        8 /* length of command description */
 #define KEYS_CMDS_PER_LINE  6 /* max number of commands per line */
 
+/* Size of the hash table the note garbage collector uses. */
+#define NOTE_GC_HSIZE 1024
+
 #define ERROR_MSG(...) do {                                                   \
   char msg[BUFSIZ];                                                           \
   int len;                                                                    \
@@ -754,6 +757,7 @@ void  edit_note (char **, char *);
 void  view_note (char *, char *);
 void  erase_note (char **);
 void  note_read (char *, FILE *);
+void  note_gc (void);
 
 /* notify.c */
 int       notify_time_left (void);