Break everything the TabWidget does

**! Do not merge this PR !**
It serves as TODO list for a rather ambitious branch.

* [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] 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)
* [ ] Bring back RenameLabels and headline callbacks
* [ ] Fix refresh calls never happening
* [ ] Fix dynamic content never being generated
* [ ] Fix several layout problems with the new Containers