Change \Nette\Forms\Controls\Button to generate `<button>` instead of `<input type="button">`

[janfejtek]

I think it would nicer to generate <button> instead of <input type='button'> with SubmitButton and Button classes to allow html content (for example bootstrap spinners ) . Also I think it could replace ImageButton class in future

BC:

• browser: I think only IE <= 10, can i use does not have the data...
• php: <button type='button'> and <button type='submit'> are valid in case anybody already changed it by extending Button class