Clear pressed keys after loosing input focus (#3532)

[thedmd]

This PR address #3532.

Implemented on all backends. Exceptions:
glut - no explicit API available to handle focus
marmalade - not supported (SDK is no longer available anywhere)
iOS - backend is very basic and handle only touches, not relevant to this PR
Android - focus event appears to not event be fully exposed in native API, not a single example was found; this is usually handled in onPause/onResume, can be considered out of the scope of this PR; using hardware keyboard isn't usual use case

[ocornut]

Thank you! Should merge soon

For future readers, note that #3532 was only reopened following changes #2445, #2696, #3751, #4377 where backends now report mouse position for unfocused window.

As thus, old backends won't be affected.
New backends which for whichever reason don't/can't use io.AddFocusEvent() can still set io.MousePos when not focused for the same results.

[ocornut]

Merged with amends: 2f40be6

• Added add changelog entries. Documented new GLFW callback as potentially breaking change.
• GLFW: added window == bd->Window test for reduce drift with docking branch.
• Made a change to NavUpdateWindowing() as following today's Pressing Alt+Shift (or Shift+Alt) will clear the focus when using NavEnableKeyboard #4439 change to use io.KeyAlt/io.KeyMods directly instead of NavInputIsPressed wrapper, we needed the equivalent of a IsReleased() (now testing old mods).
• For SDL and Allegro5 backends I changed the return value to match others. For the simple/weird reason that this return value has in fact been not defined/clarified and never used (more or less: it never worked and was never useful). So rather than pretending to have it half-work for now it's keep consistently useless.

Thanks a lot again! Amazing help.