Added command palette that allows the user to search for (and execute) commands at any time. Unbound by default. Resolves #555

- The updater can now update itself

MBF21 Game Configuration: Added missing "friendly" flag to things (thanks to hawkwind3). Resolves #703
MBF21 Game Configuration is now enabled by default

GZDoom game configuration: added support for distinct upper/middle/lower sidedef brightness levels. Resolves #694

GZDoom game configuration: added enum list for the automapstyle UDMF linedef field

UDBScript: groups all classes, objects, and methods in a new UDB namespace, usable with the new feature version 4. Updated the documentation accordingly. Resolves #679

UDBScript: Exported the classes Linedef, Sector, Sidedef, Thing, and Vertex, so that they can be used with instanceof
UDBScript: Map class: the getSidedefsFromSelectedLinedefs() method now correctly only returns the Sidedefs of selected Linedefs in visual mode (and not also the highlighted one)
UDBScript: Map class: added a new getSidedefsFromSelectedOrHighlightedLinedefs() method as the equivalent to the other getSelectedOrHighlighted*() methods
UDBScript: Sector class: added new floorSelected, ceilingSelected, floorHighlighted, and ceilingHighlighted properties. Those are mostly useful in visual mode, since they always return true when the Sector is selected or highlighted in the classic modes. The properties are read-only
UDBScript: Sidedef class: added new upperSelected, middleSelected, lowerSelected, upperHighlighted, middleHighlighted, and lowerHighlighted properties. Those are mostly useful in visual mode, since they always return true when the parent Linedef is selected or highlighted in the classic modes. The properties are read-only
UDBScript: added new example to apply textures for floor/ceiling and upper/middle/lower texture for selected map elements
UDBScript: updated documentation

Eternity Engine game configuration: removed the lightfloorabsolute and lightceilingabsolute fields from the sector flags, so that they are handled correctly. Fixes #670

Improved associations between arbitrary UDMF fields (used by the Eternity Engine game configuration)

* Updated the bundled Zandronum ACC with Zan 3.1 changes

* Add Zandronum 3.1 functions and constants to its ACS configuration

* Updated the Zandronum ACC (again), as it was missing GAMESTATE definitions

UDBScript: the methods of the Pen class now return the instance of the Pen class to allow method chaining. Resolves #662

Hexen Game Configuration: fixed a problem where the default arguments of the Death Wyvern had special names, even though they actually don't do anything in game. Also renamed the thing from "Dragon Lich" to the correct "Death Wyvern". Fixes #660

Added map scripting capabilities to UDB (#656). Documentation at https://biwa.github.io/udbscript-docs/

(G)ZDoom game configuration: added several missing UDMF fields for linedefs and sectors, so that they show up in the custom tab. Fixes #628

Eternity Engine game configuration: it's again possible to set the floor and ceiling brightness independently from the sector brightness

Game configurations: added option to enable/disable GZDoom style distinct brightness for walls/ceilings/floors. Fixes #618

Eternity Engine game configuration: fixed a problem where the Friendly and Dormant flags were not correctly pasted in Doom format.

Eternity Engine game configuration: fixed a problem where the 3DMidTex flag was not correctly pasted in Doom format. Fixes #606

Eternity Engine game configurations: added support for arbitrary map names in the default testing parameters

Boom game configuration: fixed a problem where linedef actions 264 and 265 were not labeled correctly

Negative vertical patch offsets are now handled correctly based on the game configuration. Fixes #561