Use HTML5 number fields in Views

[quicksketch]

This is a followup to backdrop/backdrop#247 and #200. We introduced HTML5 number and range elements, but there are a few places within Views that could leverage this functionality which were not included in the original PR.

Views includes a custom validation function for positive integers, views_element_validate_integer(). We should remove this entire function and any places it's used (number of table columns, results per page, etc) within Views. These places can be replaced with an HTML5 number field.

[quicksketch]

This isn't a blocker for 1.0.0, so moving to 1.x-future. We can make this change at any time.