Move frequent locations to separate XML file

[ph10w]

Summary

• store frequent locations in a separate frequent_locations.xml file when XML preferences are enabled
• load legacy frequent-location data from config.xml when the new file does not exist, allowing it to migrate on the next save
• derive the new file path from the configured config.xml location, including when EXPLORERPP_CONFIG is used
• keep registry-based preference storage unchanged

Motivation

Frequent locations are machine-specific usage data rather than portable application preferences. Keeping them in config.xml makes that file less suitable for reuse on another PC.

User impact

Users can copy config.xml between machines without also copying their frequent-location history. Existing XML configurations migrate automatically without losing their current frequent locations.

Validation

• built Explorer++ in Release|x64
• built TestExplorer++ in Release|x64
• passed all 12 FrequentLocations* tests
• passed StorageTest.ConfigEnvVar and StorageTest.FrequentLocationsFilePath