Annotate OnMouseButtonPress and related code

[lL1l1]

Description of the changes

Annotate OnMouseButtonPress and related code

Testing done on the proposed changes

read output from clicking with left/right/middle mouse buttons after using this command

local ui = import("/lua/ui/uimain.lua")
local f = function(event)
	reprsl(event)
end
ui.AddOnMouseClickedFunc(event)
ForkThread(function()
	WaitSeconds(5)
	ui.RemoveOnMouseClickedFunc(f)
end)

Checklist

• Changes are annotated, including comments where useful
• Changes are documented in a changelog snippet according to the guidelines.
• Request 2-3 reviewers from the list of reviewers and their areas of knowledge.

[4z0t]

Perhaps creating an alias for this function type would be nice.