Rename the addon to ShagaTweaks

The addon folder, toc files and title are now ShagaTweaks, and the modules
that load their own textures look them up in the ShagaTweaks folder.

The Lua namespace (ShaguTweaks) and the saved variables (ShaguTweaks_config,
ShaguTweaks_cache) keep their names, so existing settings carry over and
addons that integrate with ShaguTweaks keep working. /st also answers to
/shagatweaks.

mods/unitframes-classportrait.lua

@@ -4,7 +4,7 @@ local T = ShaguTweaks.T
 local addonpath
 local tocs = { "", "-master", "-tbc", "-wotlk" }
 for _, name in pairs(tocs) do
-  local current = string.format("ShaguTweaks%s", name)
+  local current = string.format("ShagaTweaks%s", name)
   local _, title = GetAddOnInfo(current)
   if title then
     addonpath = "Interface\\AddOns\\" .. current