tabwidget: Rewrite and relayout overview_{signalfire,tower}

Two proof-of-concept tab conversions.
tabwidget: Rewrite and relayout editor_settings

This also adds some explanatory labels and an actual heading.
tabwidget: Rewrite and relayout overview_tradership
tabwidget: Rewrite and relayout overview_enemy{building,unit,warehouse}

Entirely removes the currently redundant overview for enemy units.
To be added at a later point where such units' information displayed in
those tabs is actually different from enemy buildings.
tabwidget: Rewrite and relayout overview_firestation
tabwidget: Rewrite and relayout overview_resourcedeposit
tabwidget: Rewrite and relayout overview_settler
tabwidget: Rewrite and relayout overview_productionbuilding
tabwidget: Rewrite and relayout boatbuilder
tabwidget: Rewrite and relayout diplomacy
tabwidget: Rewrite and relayout island_inventory

This needs more love for the dynamic background icon generation still.
tabwidget: Rewrite and relayout overview_select_multi
tabwidget: Rewrite and relayout overview_{trade,war}_ship
tabwidget: Rewrite and relayout tradetab
tabwidget: Rewrite and relayout buysellmenu
tabwidget: Rewrite and relayout tab_account

This retires WarehouseOverviewTab and its layout file overview_warehouse.
tabwidget: Rewrite and relayout buildtabs
tabwidget: Rewrite and relayout place_building
tabwidget: Rewrite and relayout related_buildings
Fix build menu headline