API: Added backend cabailities for moving of a custum number of frames

loader.goto(self, energy)
loader.energySteps(self, energy)

goto(self, energy) in MainWindow

Signed-off-by: Nicola Ferralis <feranick@hotmail.com>
New button for setting the custom energy.

A toggle button is added to the right of the slider. When pushed,
a text field opens where the desired energy can be typed.
By either pushing return or re-toggle the button, the
frame is set to the desired frame.

Signed-off-by: Nicola Ferralis <feranick@hotmail.com>