Added const qualifiers is_next_to_empty_box() became visible to other files().
The size of the main window has been changed.
Checking whether the A* algorithm has been launched before starting a new game
or the same A* algorithm.
Fixed indentation in lambda expressions.
The initial node is added to open_queue without additional creation of dynamic
memory (the address of the object field is taken).
Fixed PQ_cont.erase().
IsNearEmptyBox was removed().
EqualNode moved to the Node class.

Moved friend declarations in classes

Termination of the program while the computer is solving the algorithm
(by esc key)

Convert pictures to array data (image_converter/converter.c)
Resources are located in image_converter/resources.o

The image widget is responsible for deleting the loaded part of the image
Check for correct type of selected file
Error checking after image upload

Added a dialog box about.
Checks if the selected file is correct.