Content search: improve the limit of results

In the search in the content tab (SearchingSectionPlugin) the search result limit is always set to a constant 100.

First, there is a TextSearchBuilder#setLimit() available but never used to be set from configuration.
Second, the limit of 100 is on searched document variants; then the result is mapped to handles, leaving a possible 34 handles in the list instead of the intended 100. Note that folders are included too so the visible result size will always vary.

Let's improve this: IMO easiest is to make the limit 300 and/or configurable. Searching for actual handles with the limit is a bigger change.