OpenTTD

This patch adds Filtering functions to the GUIList class.

You can specify different callback functions that are used to decide if an item would be filtered out or not, much like the sort functions work. The pointer to those functions is given to the GUIList with SetFilterFuncs(..) similar to SetSortFuncs(..). If no Filter functions are specified and/or filtering is disabled with e.g. "SetFilterStatus(false)", nothing will be filtered.

To filter the GUIList, you have to manually call Filter(const char *filter_string), where filter_string is an optional string you can pass through to the callback function(s) to decide on an item. It is advised to call this right after (re)building the list, before sorting.

If an item is filtered out, it is currently just erased from the list, so to get back filtered out items (if e.g. the filtering conditions have changed), you need to rebuild the list.

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Attached are two files, the first one (GUIList_filtering_base.diff) contains only the changes to the GUIList class to allow for filtering.
The second one (GUIList_filtering_content.diff) is an example application of the filtering in the online content window. It allows entering a keyword that the list is compared with and only items containing this keyword are displayed.