New features

Mapper device : this new kind of device allows to map existing addresses of other devices between them.
It creates a "virtual tree": when reading and writing to the address of this device, transformations will be applied.
An example of the possibilities is provided here : https://github.com/OSSIA/score-user-library/blob/master/Devices/Mapper/Mapper.qml

Changes

The states put in the last state of the top interval will now trigger on stop (#858, thanks for the report @composingcap)
Better symbol for slot grips (thanks @bltzr)
No more slot menu (#808), everything can be done by drag'n'drop nowadays
~~It's now possible to change the curvature of multiple curve segments at once (part of #764, thanks for the suggestion @evanmtp)~~ sorry, still buggy
It's possible to put processes in new slot by dragging from the object panel which can be useful if a slot has been removed for some reason
If a process is moved from an interval, and the interval has no following intervals nor end state (basically the process is alone) then the interval is entirely moved :

Fix dragging of slots which was very hard to do (it only worked on a few pixels)
Fix category of Pitch process (thanks @thibaudk)
Play-from-here was sending messages in random order.
Fix behaviour of mute when playing / stopping.
#870
#869
#868
#867
#859
Many refactors and code simplifications

NOTE: due to it being a recent feature, the format of the sub-scores saved in the library has changed a bit which may break when loading old ones.