string displaying frame counter should have constant width

[pariasm]

In each window there is a string like this: "#1 [4/16] path/to/frame/005.png"

When flipping, this string is the only way of knowing which sequence is being displayed in the window. The frame number (i.e. [4/16] should be formatted as printf("[% Nd/%d]", current_frame, total_frames); where N is the number of digits of the sequence length. In this way it does not change in width depending on the number of digits of the number.