Dialogs/dlgChecklist: Don't pollute global namespace
Dialogs/dlgChecklist: Fix indent
Dialogs/Settings/ConfigPanel: add {Borrow,Return}ExtraButton()

Replaces GetExtraButton(), and gives the dlgConfiguration.cpp core
more control.
dlgConfiguration: don't show the hourglass cursor

This is legacy from a time when opening the dialog took tens of
seconds.
dlgConfiguration: remove obsolete TODO comment

We don't want Cancel buttons.
Form/CheckBox: add default constructor and Create() method

Allow CheckBoxControl objects with no visible Window.  This will allow
avoiding dynamic allocation.
Form/TabMenu: use PagerWidget instead of TabbedControl

PagerWidget has everything we need, and there is no use for
TabbedControl's overhead.
TabMenuDisplay: add Altair hot keys
Form/TabMenu: GotoMenuPage() returns void

Nobody used the return value.
Form/TabMenu: convert enum to constexpr
Form/TabMenu: move code to OnPageFlipped()
Form/TabMenu: use PagerWidget::Next/Previous()
Form/TabMenu: eliminate attribute last_content_page

Obtain the value from TabMenuDisplay::GetSelectedIndex().
dlgConfiguration: don't focus arrow buttons on hot key
dlgConfiguration: move hot key handler to TabMenuControl::InvokeKeyPress()
Form/TabMenu: remove menu page hot key special cases

Not necessary.  If we're on the menu page, TabMenuDisplay::OnKeyDown()
will handle that.
Form/TabMenuDisplay: remove redundant check from OnKeyCheck()

The call to IsCurrentPageTheMenu() is redundant because when the
TabMenuDisplay has the keyboard focus, IsCurrentPageTheMenu() must
return true.
Form/TabMenuDisplay: initialise selected_index at the first item

Replaces the kludge in TabMenuControl::HighlightNextMenuItem().
Form/TabMenuDisplay: remove redundant check from OnKeyDown()

The call to IsCurrentPageTheMenu() is redundant because when the
TabMenuDisplay has the keyboard focus, IsCurrentPageTheMenu() must
return true.
Form/TabMenu: move Highlight*() to class TabMenuDisplay

This belongs in TabMenuDisplay because it only affects this class.