Class-based import processors and more powerful HTML import

### What does it do ?

Make import processors to class-based.
Add powerful import by JSON-like import_element.

### Why is it needed ?

Now import_element parameter from Import forms is not used at all and set to body. With this update it can be possible to set different fields and TVs of resource via JSON field.
This field is of following template:
```
{"field1":"value1", "field2":"value2", ...}
```
where fields are all fields of resource or TV names, and values are values of these fields set explicitly or jQuery-like selectors started from $.

**Example:**
```
{"parent":8, "template":2, "content":"$.content", "pagetitle":"$title" "testTV": "$.content #test-tv"}
```
It will import resources with parent=8, template=2, content= inner HTML of tag with class "content", pagetitle = value of tag "title", testTV = inner HTML of tag with id=test-tv and a child of tag with class "content".

### Related issues

Fixes #3975
#11437