Clink: all versions and old releases

- Popup lists can now be filtered.
- Press F4 in a popup list to toggle the search mode between "find" and "filter".
- The clink.popup_search_mode setting controls the default search mode.
- The clink.popuplist() function can be told to start in a specific search mode.
- Added os.findfiles() to allow getting files/directories one at a time (versus os.globfiles() which collects the entire set all at once into a table).
- Added exec.associations setting to let Executable Completion include files with registered file associations as matches (e.g. launchable documents such as "*.pdf" files).
- Fixed Ctrl-C in a popup list copying the wrong entry after having used Del to delete an entry.
- Fixed input line coloring to use color.executable also for files included by the exec.path setting, if the files have registered file associations.