Improve removal of a page type

This pull request improves the removal of a page type: when it's used
by some page, these pages are assigned to an other page type
(specified by the user) so the removal can be accomplished.
Standardization of interface and logic

Let's make things a little bit more standard for concrete5
Add a check on existing page type
Optional parameter set to 0
Dialog creation simplified
Let's use the SecirityHelper
Merge branch 'allow-select-pagetype-on-delete' of github.com:mlocati/concrete5 into merge_764_page_type_removal

Conflicts:
	web/concrete/core/controllers/single_pages/dashboard/pages/types.php
Merge pull request #1185 from mkly/merge_764_page_type_removal

Merge 764 page type removal