can configure GUI 3D depth in Settings and by custom actions

so far GUI 3D depth was hardcoded as 25% of screen width
now it can be adjusted in range <5% 50%> of screen width
either in Settings or by calling custom commands:
"gui3ddepthinc", "gui3ddepthdec", e.g. by adding keyboard mappings:
<eight>gui3ddepthdec</eight>
<nine>gui3ddepthinc</nine>