Bring Dota-2-Alt-Tab up to date

Add Bristleback, Skywrath Mage, Abaddon and Elder Titan
Update icons for Centaur Warrunner and Sniper
Update guide for Treant Protector
Change guide for Bristleback and Elder Titan in respect to boots
Add description for Tusk
Add more tips for Treant Protector, Tusk, and Bristleback
Small typo fixes