A rich text label widget that can show text marked up with help markup.
Also includes the GUI Test Window, accessible in the title screen after launching wesnoth using --clock option. It can be used as dialog template/example or as a place to test GUI2 code.
Currently for the vertical listbox only, since it does have some quirks. Removes the assert in the "show" policy checking if the callback is null. It won't be for the listbox, but doesn't really matter since it's not used.
* Redesigned the version dialog to serve as a general purpose About dialog.
* Credits button removed from title screen
* Moved community dialog contents to about dialog as new tab
Co-Authored-By: Pentarctagon
* Bugfixes to the tab_container widget
* Success indication mechanism on copy buttons
