DisassemblerGraphView zooming fix

[AntonReborn]

What has been fixed:

Previously, during zooming using the wheel or +/-, the GraphView was zoomed to the top left corner. As well the scaling was linear, which break an experience, as almost all the modern editors use exponential zooming.

What has been changed:

• In order to implement zooming to the cursor, the offset member of DisassemblerGraphView had to be adjusted correspondently. If using +/- the Graph zooms to the center of view;
• current_scale now multiples by the exponent, instead of adding the constant.

Test plan:

1. Open a graph with disassembler;
2. Try to zoom.