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.

Merge branch 'master' into navigation ...

# Conflicts:
#	imgui.cpp
#	imgui_internal.h

Fixed IsItemHovered() - part of the processing has to be done in ItemAdd() because the widget may alter clipping rectangle temporarily.

Nav: Fixed NavDown to open menu from a menu bar (#787)

Merge branch 'master' into navigation

Refactor to move the responsability of SetHovered() to ItemHoverable() - previously IsHovered(). Simpler and the parallel to ItemAdd is clearer with the new name.

Removed another msileading difference between the hovered functions (IsWindowContentHoverable() uses the root window already). Sorry for the commit spam! (making small commit to easily be able to Bisect those in case I make a mistake)

IsItemHovered(), IsHovered(): Shallow tweaks, eventually removed all the braces.

Internals: Moved IsWindowContentHoverable() in the file