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
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

This includes some wonderful hacks. Sorry.
tabwidget: Relayout base_tab
Hotfix inhabitant and main square overview

Comment out all the code and add some TODOs.
Support for two header icons in tabwidget

Specified in the same way as headline text currently is.
Still not sure how this is supposed to look and work in the end, I am
not very comfortable with passing more and more stuff as attribute.
tabwidget: Shortcut for displaying player emblem as left icon

Similar to how stances are handled. Wishlist: Player name as tooltip?