add distinct icon for Export Lib To Files

[gsokoll]

Summary

• Export Lib To Files previously reused the Export To Files icon in config.json, so the toolbar showed two identical buttons
• adds a new icon (colored book spines on a shelf with the red export arrow) drawn in the same 16x16 flat style as the existing icon set
• points the Export Lib To Files entry in config.json at the new .ico
• updates docs/toolbar.png to show all five toolbar buttons, matching the five scripts listed in the README

Testing

• config.json parses and the .ico is a valid 16x16 RGBA frame matching the existing icons
• reinstalled via install.bat and verified in CODESYS: five buttons appear and the lib export button shows the new icon (see updated docs/toolbar.png)