Nav: Fixed RenderNavHighlight() clipping, essentially revert 6ea90af. (#787)

Merge branch 'master' into navigation

Begin: Moving some code around for the benefits of the wip viewport branch.

Internals: Using ImFloor().when appropriate.

Merge branch 'master' into navigation

InputText: Added alternative clipboard shortcuts: Shift+Delete (cut), Ctrl+Insert (copy), Shift+Insert (paste). (#1541)

InputText: Moved code for checking clipboard shortcut so we can expand on it. (#1541)

Popups: Comments, reorganize bits of the header section, renamed parameters.

BeginPopup: Exposed extra_flags publicly. (#1533)

Merge branch 'master' into navigation ...

# Conflicts:
#	imgui.cpp

OpenPopup(): Always reopen existing popup. Removed OpenPopupEx() bool reopen_existing which is always true. This also makes the public API on par with OpenPopupEx(). (#1497, #1533)

Popups: Gently handle the user mistakenly calling OpenPopup() every frame. (when reopen_existing is true). (#1497)

Internals: Popup: Renaming fields.

Popup, Menus: Tweaks and comments.

Internals: Popup: Explicitely setting up ImGuiPopupRef reduces confusion.

Popups: Revert aca23fd (Oct 20, 2017). Because 1) I can't seem to find a default. 2) The if is definitively faulty and would have been all true. 3) It looks like possibly the following commit 6ab737a could have made this unnecessary. Not absolutly certain. (~#439)

Popup: BeginPopupContextItem(), BeginPopupContextWindow(), BeginPopupContextVoid(), OpenPopupOnItemClick() all react on mouse release instead of mouse click. Note that they don't use the full ButtonBehavior() or tracking aabb on both click and release. Applications I've tried seems to behave inconsistently there but on-release-without-tracking is both fairly common and doesn't require extra code for the id tracking. (~#439)

Internal: ButtonBehavior: Tweak to update g.ActiveIdClickOffset more consistently

Merge branch 'master' into navigation + fix previous merge ...

# Conflicts:
#	imgui.cpp

Internal: Reworked FocusFrontMostActiveWindow() so it fits the Nav branch usage as well.

Merge branch 'master' into navigation ...

# Conflicts:
#	imgui.cpp

Internals: Popup: Separating MousePosOnOpen and PopupPosOnOpen. They are equal in the master branch but different in the navigation branch.

Internals: renamed FocusPreviousWindow() to FocusfrontMostActiveWindow()

Internals: BringWindowToFront() iterate front to back, faster in majority of use cases.

Revert "Internals: Added LastActiveId, LastActiveIdTimer. (#1537)" Will come up with a better design later. ...

This reverts commit 007f403.

Internals: Added LastActiveId, LastActiveIdTimer. (#1537)

Merge branch 'master' into navigation ...

# Conflicts:
#	imgui.cpp

Merge of minor left-overs from private work branch to reduce drifts. Should be functionally a no-op.

Fixed non-popup child windows not honoring the HiddenFrames flag (can't see a reason). Docking relies on this.

Internals: Settings api tweaks