Add notification counters to bottom tab

A counter now increments in the bottom bar for unfocused windows showing how
many unread messages are pending. Tabs with no pending messages show [*]
instead of their index (showing the index is useless and somewhat confusing)

src/windows.h

@@ -167,6 +167,7 @@ struct ToxWindow {
     char name[TOXIC_MAX_NAME_LENGTH + 1];
     uint32_t num;    /* corresponds to friendnumber in chat windows */
     uint8_t index; /* This window's index in the windows array */
+    unsigned int pending_messages;  /* # of new messages in this window since the last time it was focused */
     int x;
 
     WINDOW_TYPE type;