List multiselection

[elexis]

As reported by cc, it would be comfortable to the user if one could select multiple items of the replay- and savegame list and delete them with one delete button- or hotkey press.

The COList.cpp file would have to be extended. If this is implemented, one should be able to

• select all items between the most recently selected item and the next selected item by holding the Shift key and
• adding items individually to the selection by holding the Control key

The replay/load buttons of the according GUI dialogs would have to account for multiselection as well.

[elexis]

In 23020:

Remove virtual inheritance from the codebase, refs rP73, rP141 rP290.

Remove workaround in rP22596 / D2136, the pointer from IGUIObject to IGUITextOwner that avoided the dynamic_cast.
Have the GUIObject implementing class take responsibility of calling the virtual methods in the base classes.
Leaves the code in a clean state that can serve as an example for new GUI object classes, such as D1346, refs #4683.

Differential Revision: ​https://code.wildfiregames.com/D2325
Comments By: Vladislav on IRC 2019-07-30, 2019-08-22, 2019-09-20-23
Tested on: clang 8.0.1, Jenkins

[Stan]

Backlogging due to lack of progress and Silier retiring.