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

IsItemHovered(), IsHovered(): Shallow tweaks to make them more symetrical. Removed one set of braces.

Internals: ButtonBehavior(), IsHovered() moved the responsability of flatten_child higher level to ButtonBehavior() - not super sure about the elegance of this

Nav: Fixed merge cc26db8

Merge branch 'master' into navigation

IsItemHovered(), ItemAdd(): finishing cleaning up, moved the code to IsItemHovered() so ItemAdd() is more lightweight and the two IsXXXHovered functions are now very similar, making their differences less confusing.

ItemAdd(): more re-ordering of tests to match IsHovered() + comments