default sorting and style attribute

[iamkallolpratim]

[Nayjest]

Hi!

1. GridConfig now has two getPageSize() methods.
2. There is no protected $defaultSort property and no use-cases for getDefaultSort() (it should be used in some way in code if it's added there).
3. What is reason for replacing .btn-default to .btn-secondary?
4. Please create separate pull requests for separate features, it will significantly increase chances of success.