It is much more natural than custom methods for this very common
operation.

If you have the setting "Emulate middle mouse button" turned on, a click
and drag can fail if it is done very quickly. The position of the
initial click will be incorrect in such a case because the timeout will
delay events.

Not every mouse has three buttons e.g. laptops. Some OS might not
have support for middle mouse button emulation.

This commit adds emulation for middle mouse button when pressing both
left and right mouse button simultaneously.