- fixed bug with incorrect textures packing: in Packer::run() after
  insertion new zones "current zone" will be changed, it need to get
  again "current zone" from the zones matrix by the same row and col indexes;
- deleted Packer::canPackTextureByWidth() and Packer::canPackTextureByHeight()
  methods;
- move AtlasWidget class definition and realization from mainwindow.h
  to the mainwindow.cpp, in mainwindow.h added forward declaration of the
  AtlasWidget class.

Signed-off-by: Oleg Matiyuk <olgerd27@gmail.com>
Textures can packing into the atlas.
In this version packing algorithm not changed.
Some changes current commit:
- deleted MainWindow::sizeInfo;
- use 20 test textures for packing in an atlas 256x256;
- deleting many debug messages;
- show zone matrix as debug message after every succesfull texture packing;
- show some debug messages about packing process;

This version have a bug - when textures, bacause of its size, cannot pack,
then appear the error in algorithm when try to get with invalid index item
from zhe zone matrix - the deque exception:
invalid deque<T> subscript (out_of_range).
Because of this bug cannot to run pack algorithm again to pack not packed
textures.
It's need to fix this bug in the nexts commits.

Signed-off-by: Oleg Matiyuk <olgerd27@gmail.com>