po: update German translation
po: update Slovak translation
dlgStartup: remove landscape XML file

No difference to the portrait mode file.
ButtonRenderer: add method GetLook()
Form/CustomButton: include cleanup
Form/Button: eliminate attribute "look"

Obtain the ButtonLook from the ButtonRenderer, which manages the
attribute anyway.
Form/SymbolButton: include cleanup
Dialogs/Message: eliminate class ModalResultButton

The standard WndButton can do this already, by passing the WndForm as
ActionListener.
Form/Button: pass ButtonLook to constructor

This class doesn't need the whole DialogLook.
Form/Button: add empty constructor, method Create()

Allow WndButton instances that will be created later.
Form: remove unused macro OPENCLOSESUPPRESSTIME
WidgetDialog: support Widget::SetFocus()

Add virtual method WndForm::SetDefaultFocus() and let WidgetDialog
implement it.
dlgStartup: don't use XML layout
Dialogs/XML: remove "filereader", "time"

Unused.
Form/Util: remove unused functions
Form/Util: convert runtime nullptr checks to assertions

These controls must exist.  For optional controls, there are special
functions.
Form/Util: disable rounding in LoadOptionalFormProperty()
Form: pass rectangle to ReinitialiseLayout()
Form: use GetPosition() instead of GetLeft(), ...

The latter methods are not available on GDI.  GetPosition() is faster,
because it's just one method call instead of four.
Form: enable the method ReinitialiseLayout() on all targets