change GetFilePath param from int to IntPtr

[molsonkiko]

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Previously the GetFilePath method of NotepadPPGateway was unusable in 64-bit Notepad++ because it accepted an int parameter, but the buffer id (the IdFrom parameter of the NPPN_FILEBEFORECLOSE notification) could be 64-bit.

Also added a new plugin command to Demo.cs illustrating how tracking the NPPN_FILEBEFORECLOSE notification allows the plugin to keep a list of all the filenames that were closed this session.

[kbilsted]

merge conflict