Include tool changes in time estimate

[AndyEveritt]

Is your feature request related to a problem? Please describe.
The time estimate doesn't consider the time it takes to change tool. Reprapfirmware handles tool changes with macros so it is impossible for the slicer to calculate the tool change time, however the time should be fairly constant for a tool change on a given printer.

Describe the solution you'd like
Tool change time for macro based tool changes should be something like:
base tool change time + time to heatup from standby temp to active temp
The standby and active temps are know by the slicer and may already be handled by the time estimate for heating up the nozzle currently? If not maybe this could be ignored for now.
Adding a default tool change time variable to the slicer would atleast give a notably more accurate time estimate for multi tool prints.

Describe how it would work
User enters optional tool change time, time estimate takes the original estimate and adds the number of tool changes * tool change time. If the user leaves the time empty (for toolchange scripts handled by slicer), then this setting is ignored.

Describe alternatives you've considered
Some people do the tool change macros in the slicer, personally I do not like this solution as the duet handles the tool changes much more cleanly which simplifies the gcode file and improves printer maintainability.