This way the save dialog gains the sortable list and details section feature from rP19351 / D246 by removing a folder of code, refs #4413.
Fixes a style warning about the old dialog using a style for an 'olist' type for a 'list' type, refs rP22792 / D2232.
Subsequently move the gui/common/ helper file to the page code, meaning it's not loaded for every GUI page needlessly anymore.
Fix map preview aspect ratio.
Use object-oriented code for the moved savegame code, refs #5387, but conserve the rest of the code to limit the changes for auditability.
Differential Revision: https://code.wildfiregames.com/D2290
Tested on: Jenkins