Fix image navigation order: natural sort instead of lexicographic

[Slashxdd]

Fixes #146

Problem

When navigating through images in a folder with arrow buttons, files are ordered lexicographically: 1, 101, 2 — instead of the expected natural order 1, 2, 101.

Root cause

GetCurrentFiles() builds the file list from Directory.GetFiles() and returns it as-is, with no sorting applied. Directory.GetFiles() returns files in filesystem/lexicographic order, so multi-digit filenames end up out of order.

Fix

Sort the list in GetCurrentFiles() using StrCmpLogicalW from shlwapi.dll — the exact same Windows shell function that File Explorer uses for its own file sorting. This guarantees the navigation order always matches what the user sees in Explorer.

[DllImport("shlwapi.dll", CharSet = CharSet.Unicode)]
private static extern int StrCmpLogicalW(string x, string y);

// in GetCurrentFiles():
arlist.Sort((a, b) => StrCmpLogicalW(Path.GetFileName(a), Path.GetFileName(b)));

One line added to GetCurrentFiles(), one DllImport declaration, one using System.Runtime.InteropServices.