Add Text: Align to mouse position

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Note: This suggestions has been made by XnView user G.S.

When adding text, it's good to have the alignment. It might be a good idea to change the insert position depending on the alignment:

- When using left aligment, it would be good if the mouse indicated the left position.
- When using center alignment, it would be good if the mouse indicated the center position
- When using right alignment, the mouse should indicate the right position.

This way, text could be inserted more precisely. To make the difference clear, the cursor shape might change, too.