FR: Datatables class should have configuration of pageLength

[djoris]

Setup

• SMW version:3.2.3 (5619c2d), but also older versions
• SRF version: 3.2.0 (ecbf369), but also older versions

Issue

When using datatables, the editor can't set a default pageLength (pagination size). Every datatable has a default of 10 rows per page. There are plenty use cases where a larger amount is desired.
It seems straightforward enough in the Javascript library: https://datatables.net/reference/option/pageLength

STR:
see https://www.semantic-mediawiki.org/wiki/User:Joris/datatables:

• format=table
• class=datatables
• limit=20
• the actual number of results is 16
• the first 10 are shown when loading the page

Expected result:
A parameter should be optional for SRF table, eg 'pagelength', where the editor could change the number of rows. This should change the number of rows shown when loading the table, until the user selects a different number of rows.