Break everything the TabWidget does

**! Do not merge this PR !**
It serves as TODO list for a rather ambitious branch. Base branch is `unify-tabs` resp. `tabw`.

See [`tabwidget`] (https://github.com/unknown-horizons/unknown-horizons/wiki/Tabwidget) for more details.

* [x] Create a custom widget for our tabs
* [x] Dynamically load tabs into one parent tab widget
* [x] Fix hackish placement of tab buttons and diverging tab-container sizes
* [x] Templating to use dynamic headlines with attribute access
* [x] Convert all xml layout files to the new format. Still missing:
 * `content/gui/xml/ingame/tabwidget/overview/overview_groundunit.xml`
 * `content/gui/xml/ingame/tabwidget/farm/overview_farm.xml`
 * `content/gui/xml/ingame/tabwidget/overview/overviewtab.xml` (check what this is used for)
* [x] Fix refresh calls never happening
* [x] Fix ship overview crashing
* [x] Icon spots on left/right side above heading (actual images defined via python on per-tab basis for now)
  * Also make build menu toggle be a header icon (needs working `capture`)
* [ ] Bring back RenameLabels and headline callbacks
* [ ] Measure and compare performance (wrt. initial refresh call instead of show)
* [x] Make translations work (some crash when changing language too)
* [ ] `hide` ambiguity with switching tabs, issue parent relations (click on tab A, tab B, tab A again and it's empty)

Optional:
* [ ] Make `TabContainer` width an attribute for xml
* [ ] Continue work on decoupling `BuildingTool` gui (move tabs to `tabs/`, make tabs more tab-like)
* [ ] Fix layout problems with the new Containers