OverlayButtonWidget: make all overlay buttons same size

The Menu overlay button was using a different size than
the zoom buttons
MapOverlayButton.cpp: refactor - move OverlayButton class to this file
MapOverlayButton: refactor combine MapOverlayButton with OverlayButton
ScreensButtonWidget.cpp: remove redundant virtual functions
ZoomOutButtonWidget: remove redundant virtual functions
GlueMapWindowOverlays.cpp: use MapOverlaysButton::GetStandardButtonHeight()
GlueMapWindowOverlays.cpp: remove "clear_border_width"

rely on sizing in MapOverlayButton::GetStandardButtonHeight()
GlobalFonts.cpp: add map_overlay_font
MapOverlayButton: add SetText() to show text instead of image

Text scales better than icons with high pixel density screens like
the HTC one
MainMenuButtonWidget, ZoomIn, ZoomOut, Screen: use text instead of icon
NEWS.txt