Internals: SetActiveID window cannot be NULL

Nav: commiting shallow bits to reduce noise from working copy/upcoming commit.

Nav: Extract part of NavUpdate() into a saner NavUpdateWindowingTarget() (#787)

Nav: Made NavWindow always refresh from NavId so we can lazily retrieve the window for user functions that don't have it. This is not required by current commit but I'd rather test it earlier. Idea: eventually if we switch to 64-bit identifiers we could reserve e.g. 20 bits to store a simplified window identifier so we can always retrieve a window from an id. (#787)

Nav: SetActiveId() uses SetActiveIDNoNav() to avoid duplicating logic. (#787)

Nav: Extract part of NavUpdate() into a saner NavScrollToBringItemIntoView() (#787)

Nav: More consistently drawing a (thin) navigation rectangle hover filled frames such as tree nodes, collapsing header, menus. (#787)

Merge branch 'master' into navigation

Rnamed AlignFirstTextHeightToWidgets() to AlignTextToFramePadding(). Kept inline redirection function (will obsolete).

TreeNode: Added ImGuiTreeNodeFlags_FramePadding flag

Nav: Renaming NavInitDefaultRequest -> NavInitRequest

Nav: Moving code next to its peers

Nav: Caching into g.NavAnyRequest to minimize hot path cost (and so we can add many more request sources)

Nav: Minor tweaks in NavProcessItem() used by ItemAdd()

Nav: Minor tweaks also toward removing processing from ItemAdd()

Internal: ItemAdd() minor shallow tweaks

Merge branch 'master' into navigation ...

# Conflicts:
#	imgui.cpp
#	imgui_internal.h

Internal: ItemAdd(), IsClippedEx(): removed unnecessary indirection for ID parameter

Removed extraneous calls to GetCurrentWindowRead() when we already have a ImGuiContext. Internal RenderXXX functions also don't need to write into the Accessed field at all.

Merge branch 'master' into navigation

Nav: Shuffled code in ItemAdd() so that NavProcessItem() may have access to LastItemId (will be required by tabbing)

Nav: Minor midying up and comments,

Nav: Removed possibly redundant test prior to calling NavProcessItem() from ItemAdd()

Nav: Moving code next to its peers

Nav: Moving the big chunk of code from ItemAdd() to NavProcessItem()

Added ActivateItem(), GetItemID() functions.

Nav: Tidying up activation mechanism so that setting NavActivateId can trigger buttons.

ColorButton: Reduced bordering artefact that would be particularly visible with opaque Col_FrameBg and FrameRounding enabled.

TextUnformatted(): minor tweak

Made AlignFirstTextHeightToWidgets() saner and faster (still disliking its name very much!)