Proof of concept for tabwidget template

TabContainer, headlines automated, dynamic attribute access

TODOs: RenameLabel support, icons below minimap (left/right area)
Rename methods for more clarity

Expose _show_tab as show_tab from now on. It found sufficient (even if
not always hack-free) distribution throughout our codebase to justify.
Add settlement.name and tier.inhabitant_name properties

settlement.name is a shortcut to its NamedComponent, intended for use
with the new TabWidget, but probably not limited to that.

inhabitant_name is a property of tier overview tabs at the main square.
Remove WarehouseOverviewTabs from warehouse, storage

Those had almost no content and were too small to display all tab
buttons in their tab area. Now sporting account tabs instead.

This also changes the icon of AccountTab to the overview tab one!
tabwidget: Rewrite and relayout mainsquare_inhabitants
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,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

Also fixes an inheritance bug.
tabwidget: More inheritance for ship overview tabs

Because we're cool like that.
Note that the plain ShipOverview tab does *not* specify a widget xml, so
it cannot be used as stand-alone tab!
tabwidget: Rewrite and relayout tradetab
tabwidget: Rewrite and relayout buysellmenu