Add tree object
Update changelog
Add button:GetDown
Call object:GetDown in button drawing function
Update changelog
Fix #129
Check type of icon argument in treenode:SetIcon
Add treenode:GetIcon
Add treenode:SetText and treenode:GetText
Add treenode.OnOpen and treenode.OnClose event callbacks
Add missing treenode method comments
Add treenode:GetOpen
Update changelog
Fix typo in changelog
Merge branch 'master' into feature/toggle-button

Conflicts:
	objects/button.lua
	skins/Blue/skin.lua