Make container for main menu buttons scrollpanel

[luttje]

First of all: Thanks for the quick merges of my PR's recently 👍 and excuse these tiny PR's over the past few days. Let me know if you'd like a break from them and I'll refrain from making more PR's for a while

This PR fixes not some main menu options being inaccessible when screen height is too 'small' for the number of buttons to display.

hook.Add("CreateMenuButtons", "test", function(tabs)
	tabs["test1"] = function(container)
		container:Add("MyTestPanel")
	end
	tabs["test2"] = function(container)
		container:Add("MyTestPanel")
	end
	tabs["test3"] = function(container)
		container:Add("MyTestPanel")
	end
	tabs["test4"] = function(container)
		container:Add("MyTestPanel")
	end
	tabs["test5"] = function(container)
		container:Add("MyTestPanel")
	end
	tabs["test6"] = function(container)
		container:Add("MyTestPanel")
	end
	tabs["test7"] = function(container)
		container:Add("MyTestPanel") -- overflows outside the panel on my resolution of 1600x900
	end
end)

Before this PR:

After:

After with fewer menu items:

Tested this in the skeleton gamemode. Scrolling works great with both the mouse wheel and that scrollbar grip.

[alexgrist]

Thank you for writing easily easy to read and understand PR descriptions and providing before and after images, it makes merging these so much quicker and easier.