Changes in Options for "Section Template" field for Sections

Currently it's confusing when we have EMPTY field which works as Inherit from Parent. Often users won't put anything there and it will lead to a Fatal Error since NO design template specified.

Propose to do the following:

1. When Adding NEW Top section (ParentId = 1), we'll list only existing designs (taken from drop-down) defaulting to the template specified in cms_DefaultDesign varianble.

This variable will be defined under Admin->Configuration->Website->Advanced: Website Settings as a text field and will default to "platform/designs/general"

Place variable after:[10.05] Config_Site_Time

Variable name will be la_prompt_DefaultDesignTemplate (already in DB) – Default "Section Template".

I'll add/change it myself in a language pack in a different task.

2. When Adding or Editing the Section of 2+ level deep we'll have list of physical template designs + empty value named [Inherit from Parent] (should be fist selection)