Dynamic Z-Layer System For User Interface

[AMastryukov]

Right now, we are statically creating elements on the UI (alivehud, generalhud). RadarDisplay is one of these elements, and it needs to be below certain HUD items (such as the crosshair, the inspection menu, etc).

The problem is that RadarDisplay is directly linked to an item (ttt_radar). If someone wishes to add another item (e.g. "Hunt" item that shows other players through walls briefly), they will need to edit the alivehud statically and add a custom HUD element that renders other players on the UI somehow.

This needs to be done dynamically instead, so we want to create a system where there are specific z-layer presets of some kind that players can choose from (e.g. item-related stuff should always go under the inspection menu, crosshair should almost always be on the bottom layer, etc).