search: Allow grid using search providers to specify maximum of results

When GridSearchResults::_getMaxDisplayedResults() is called after the first character has been entered in the search, GridSearchResults has no children yet. _getMaxDisplayedResults() tries to determine the maximum number of search results based on how often the largest child would fit into the allocated size or -1 (i.e. no limit) if there are no children. So for the initial search there is no limit and in the app search all matching apps get added as possible results, which due to the search term being only a single character is almost all installed apps. This now causes allocation to be run for all these results, despite the vast majority of them never being visible, which on slower machines can cause noticeable delays before the search results are displayed.

This now adds the ability for search providers to specify a maximum number of results that gets used instead of -1 when specified. By being provider specific this means extensions implementing their own providers will not be affected by this.

Further this sets the maximum for the app search provider to 6 as per the current designs.

Closes: #7155 (closed)