Nav: Allow to collapse tree nodes with NavLeft and open them with NavRight (#787, #1079)

Nav: Disabled the final axial check when considering candidates in most situations except menubars. ...

It's definitively undesirable inside Menu as we want to catch nav request failures reliably.
I think it may be considered as an option if we find this desirable i some circumstances. Right now ideally I'd remove it totally but with current scoring setup, without it we can't easily reach the Window Close button. (#787)

Nav: It's now possible to navigate sibling of a menu-bar while navigating inside one of their child. If a Left<>Right navigation request fails to find a match we forward the request to the root menu. (#787, #126) ...

Currently the sibling menu is isn't automatically opened, that's still left to it (and even that can be anoying in Windows when the first menu-item is a child menu)

BeginMenu(): fixed logic to distinguish vertical menu from one layed out in a menu bar. Makes MenuItem() in a regular window behave more consistently, and this will be also needed by upcoming menu-navigation changes in the nav branch. (#126, #787)

Nav: Comments

Nav: Fix typo from 9712a81

Nav: Minor tidying up (adding local variables to be a little more sane).

Nav: Renamed internal field.

Nav: Rectangle rectangle stored per window and per layer as well. Makes things simpler, allows enable us to visualize more data.

Nav: Marked misleading/broken code that will never execute.

Nav: Fixed menuitems/selectable on menu layer (layer 1) from storing themselves in wrong layer of LastNavIds when hovered.causing inconsistencies and bugs when hovering menu with mouse then pressing ALT to return to layer 0). ...

NB: this is not a new bug introduced by the previous commit (which is related to nav layers), bug has been there for a while. (#787)

Nav: Store per-window last nav id also per-layer so we can easily query them for menu navigation code. (#787)

Merge branch 'master' into navigation ...

# Conflicts:
#	imgui.cpp
#	imgui_internal.h

BeginMenu: Shuffling braces around to clarify flow

Nav: Fixed SetItemDefaultFocus from stealing default focus when we are initializing default focus for a menu bar layer (#787)

Nav: Support for fallback horizontal scrolling with PadLeft/PadRight (nb: fallback scrolling is only used to navigate windows that have no interactive items). (#787)

Nav: Commiting some better organized Debug helper because this going to stay for a bit.

Removed extraneous test.

ColorPicker4: Use nav flag to provide a slighly better experience when using navigation (#787, #346)

Nav: Added ImGuiItemFlags_NoNav item flag

Nav: Re-arranged ItemAdd() to maximize early out (#787)

ButtonBehavior: Tidying up.

Nav: Internal nenaming

Nav: Changed internal flag to NoNavDefaultFocus to be false by allow, and more consistent (#787)

Nav: Fixed ButtonBehavior mistakenly setting active id when the Activate button is held and we have a new NavId, which affected browsing some popups (#787)

Merge branch 'master' into navigation ...

# Conflicts:
#	imgui.cpp
#	imgui_internal.h

Begin: Shallow tweak to minimize diff with nav branch

Merged from Navigation branch: ImGuiItemFlags_SelectableDontClosePopup flag

Added VerticalSeparator() entry point in imgui_internal. Seperator() in an horizontal layout context still does that.

Separator(): Tweak Logging so that the separator text is aligned according to tree padding.