[NOSQUASH] Use our GUIElement classes everywhere

[Desour]

• Fixes Prefer internal GUIElement/formspec classes whenever possible  #12366.
  • Note though that we can't yet delete the gui classes in irrlicht that we don't use, because irrlicht itself still uses them for other gui elements.
  • Also note that CGUIEditBox is not replaced by GUIEditBox, as latter is an abstract class, and I also don't know what it does different.
• Additionally, the globals guienv and guiroot are removed.
  • guiroot: We don't actually need to create a root gui element (was a big static text). The guienvironment already provides a root element.
  • guienv : The guienv is always reachable through the renderingengine, so it's redundant.

• Least and first, touchscreengui is made to compile (had a missing header and a const in vector elem type).
• The color (BgColor) of buttons is now initialized.

To do

This PR is a Ready for Review.

(I recommend reviewing commits separately.)

How to test

• Compare before/after of:
  • Any scrollbar.
    Note how the arrow buttons are now brightened on hover.
    Styles, however, do not affect the buttons. (As expected.) See below for test formspec.
  • The volume change menu.
  • The key change menu.
  • The touchscreengui. (Compile with -DENABLE_TOUCH=1.) (Haven't tested on an actual phone.)

Formspec for testing possible style issues with scrollbar (set via devtest node meta editor):

formspec_version[6]
size[10,10]
style[*;bgcolor=red]
button[1,1;2,1;btn;Button]
scrollbar[1,3;0.25,3;vertical;scrbar;0]

[sfan5]

did some quick tests, seems good