Implement simple move tool

[gargsms]

I had been experiencing troubles getting the text positioned exactly where I wanted it, so went ahead and implemented the move tool.

There is a shortcoming to this, however. Data is lost as the drawings move beyond the edge of the canvas.
Let me know if there is a workaround for that.

[viliusle]

First, thank you for trying make it better.

I checked your changes. But there are already similar tools:

1. first tool "select tool" - can be used to move layer with mouse, or with keyboard arrows (shift, ctrl can be used too)
2. second tool "select area tool" - part of layer can be selected and moved/transformed.

i will reject merge, let me know if something was missed.

p.s. i know issue with edges. System do not allow saving data outside, so for now it is normal behavior. Plus you can use it to erase layers left/right/top/bottom sides. Fixing it would be easy.

[gargsms]

Now that you mention this, I realize that I completely overlooked this.
The PR feature is already there. Better I close this.