This is a major update. The following major new features were added:

- VMM: new APIC and I/O APIC implementations that result in significantly improved performance in certain situations (for example with networking, bug #15295)
- VMM: added support for Hyper-V paravirtualized debugging of Windows guests
- VMM: emulate even more MMIO and shadow pagetable exits without going back to user mode
- GUI: overall migration to Qt5 (bug #11775)
- GUI: passive API event listener improving the VM GUI performance and response time
- Audio: added HDA (High Definition Audio) support for newer Linux guests
- Audio: added on-demand timers which should improve the overall performance and reduce the CPU consumption
- Audio: more fine-grained volume control for the AC'97 emulation, which now also takes the master volume control into account
- better support for Python 3

In addition, the following items were fixed and/or added:

- VMM: activate the x2APIC by default for Linux guests
- VMM: many more fixes
- GUI: the Detach UI action is now a part of the VM Machine menu
- GUI: reworked session information window
- GUI: the new VM wizard now allows to choose the VM location for the VM which is being created
- GUI: fixed location of touch events in unscaled HiDPI mode (bugs #14366, #14367)
- GUI: various bugfixes and internal cleanup
- Storage: use fallocate() or similar if present to speed up creating fixed-sized disk images (bug #5967)
- Storage: implemented NVMHCI-compatible storage controller
- Audio: various bugfixes and infrastructure improvements
- E1000: implemented interrupt throttling for performance improvements
- EFI: many improvements, for example allow to boot from USB
- OVF: improved handling of signed manifests
- API: Windows XP guests and Windows 2003 server guests now default to the E1000 T Server network card
- VBoxSVC: fixed several memory leaks when handling .dmg images
- Installers: ship VBoxBugReport
- Windows hosts: reworked COM proxy (e.g. bug #8298)
- Linux installers: no longer rely on DKMS for module rebuilding
- Linux Additions: fix a problem when updating the initramfs after the guest modules are compiled (bug #15579)
- Linux Additions: try to fix a problem where the OpenGL libraries where loaded by the X server when we only support X clients (bug #15574)