Outputs for vgui_screen

[Petercov]

Addresses #263

I had this already done from playing with the SiN Episodes maps in my mod. Any output on the vgui_screen entity that starts with a tilde (~) will be fired by the panel recieving a vgui command from its children that matches the string following the tilde.

[samisalreadytaken]

Since this is only used for vgui screens, instead of adding never-used CBaseEntity::HandleEntityCommand(), wouldn't it make more sense to name the command something like "VGuiScreenCommand", checking entity type (classname or dynamic cast) here then handling it in CVGuiScreen::OnVGuiCommand(), keeping everything localised?

[Petercov]

I intended that handler function to be generic, so other entity types could easily use it.

Edit: I originally designed that function for this entity: https://github.com/Petercov/Source-PlusPlus/blob/ec8c0337670a4de320502a8cc2855b3bb760f5ec/src/game/server/Human_Error/hlss_combine_interface.cpp#L218-L249

[Yankumo-Koishi]

Could you clearly describe how it fires inputs to entities, I checked the files in Sin Episode before but I'm still confused

[se10leptakamara]

Thank you so much for this PR, I was finally able to receive outputs from my VGUI Screens! This is a lifesaver, I'd love to see this merged to Mapbase in the future.