XGUI settings fixes and improvements

Getting the XGUI server and sandbox pages a bit more up to date.

[ADD] XGUI: Support new sandbox and server settings in the Server and Sandbox settings tabs, removed deprecated settings.
[ADD] XLIB: New helper element "Scrollable panel".
[ADD] XLIB: Some helper elements now suport DOCK layout.
[FIX] On listen servers, using the slider on some gmod server settings would cause the setting to constantly jump around. (Fixes https://github.com/TeamUlysses/ulib/issues/73)
[FIX] XGUI: "Keep AI Ragdoll" setting replaced with "Keep Corpses", which should work now.
[FIX] XGUI: Sandbox "Persist" setting now supports setting the persistence file name.
[FIX] XLIB: Alpha slider bar no longer saves decimal, no longer backwards while typing in a value.
[CHANGE] XGUI: Update sandbox limit definitions and slider maximums, removed deprecated limits.

TeamUlysses / ulx