Limit result with SQL if property 'sortBy' is used

[halftrainedharry]

The way the search is currently implemented can be very inefficient.
By default, all matching resources are returned by SQL and then array_slice() is used in PHP for the paging.
This makes sense, if the matching resources are sorted by relevance in PHP (which is the default), but is unnecessary if the &sortBy property is used (and the results are already sorted in SQL).

---

If "SimpleSearch" is used on a bigger site in combination with a commonly used word as the search term, the amount of matching resources can create a "memory limit error".
https://community.modx.com/t/diagnosing-simplesearch-memory-limit-error/5700

This PR adds a way to avoid this (while still allowing the use of the relevance sorting in PHP for smaller search results).

[[!SimpleSearch?
    ...
    &maxCountPhpSort=`200`
    &fallbackSortBy=`createdon`
]]

With a snippet tag like this, the relevance sorting in PHP is used, if the total amount of matching resources is less than 200.
For a larger amount of matching resources, the result is sorted and limited with SQL (using the value from the property &fallbackSortBy for the sorting).