A 'Single Instance only' option for XnMP

[budz45]

'Single instance only' feature:

When I dragged n dropped an image file directly onto the xnview.exe, viewed it and then drag n drop another separate image file to xnview.exe then XnView MP would open two separate window's. I thought that second image file would just open in a next tab but then I realized that XnMP is lacking this option of a 'single instance'.

So this feature is needed which would make sure XnView MP would NEVER open another separate window and would keep all newly opened files in tabs

[xnview]

Please check XnViewMP 0.30

[budz45]

Thank you so much, feature/fix added