Remove hard-coded limits restricting the maximum number of sensors

List of changes:

• t_labelledlevelbar *panels[MAX_NUM_CHIPS][MAX_NUM_FEATURES] -> std::map
• GtkTreeStore *myListStore[MAX_NUM_CHIPS] -> std::vector
• Remove the constants MAX_NUM_CHIPS and MAX_NUM_FEATURES
• t_sensors* -> Ptr<t_sensors>
• t_sensors_dialog* -> Ptr<t_sensors_dialog>
• Move NULL pointer checks from run-time to compile-time
• Remove redundant @param in the documentation of functions
• Add xfce4::put(), because std::map.insert_or_assign() is a C++17 feature