Value Ladders

This patch implements value ladders for number and number-slider buttons.

About Value Ladders
Now, what are value ladders and how will they work?
Value ladders are small pop-ups from which you can edit the individual digits of a button's value by clicking and dragging the according “step” of the ladder.

Current Design (UI + UX)
A ladder can be created, by 'alt+LMB' (click+release). You can then manipulate the values/steps by 'LMB' (click+drag+release). Holding down shift while doing this, makes the value change slower than before, providing more control.

You can repeat that on as many steps as wanted.
With a click on 'LMB' or 'Enter' the changes are confirmed and the value ladder gets closed. It is always possible to cancel the process and reset the original value by pressing 'ESC' or 'RMB'.

Support for Multi-Number Button Editing
The new value ladders do also support multi-number button editing (D270). You simply need to 'alt+LMB' (click) and drag over the wanted buttons, to edit them all at once.

Further Design
Please do not request design changes here, as this is mainly for code review. A design task for this will follow, soon.

Note: Patch depends on D759. Make sure to patch it, or this will fail.