Fix #87 - Add backdrop mixin to GameTooltip

[MotherGinger]

Included in this PR

• Fixes #87
  • Apply the proper mixin to gametooltip when skinning the dropdown frame
  • The latest WoW API has removed backdrops on frames by default, and now requires that you extend the proper backdrop template, or use a mixin to reapply the backdrop interface.
  • Reference the NineSlice subframe for backdrop settings on GameTooltip
  • Remove deprecated XML frame <Backdrop> templates and convert them to the latest API
  • Default the dropdown to a global blizzard backdrop when we can't find one on the tooltip
  • There are some odd cases where it seems that GetBackdrop isn't actually defined on the NineSlice. If we can determine why we might be able to improve this to avoid any chances of ignoring reskinned tooltips (for instance from AddonSkins).

Results:

[murphy1312]

does not have a background color for me.

[murphy1312]

For now I fixed it locally like this:

-- Make the frame match the tooltip

local function InitializeFrame(frame)
--[[    from https://www.wowinterface.com/forums/showthread.php?t=58109]]
    frame:SetBackdrop({
        bgFile = "Interface\\DialogFrame\\UI-DialogBox-Background",
        edgeFile = "Interface\\DialogFrame\\UI-DialogBox-Border",
        tile = true,
        tileSize = 32,
        edgeSize = 32,
        insets = {left = 12, right = 12, top = 12, bottom = 11},
    })
    frame:SetScale(GameTooltip:GetScale())
end

does not look great, but it's usable

[MotherGinger]

does not have a background color for me.

Good catch, I was wrong we will need to still respect the NineSlice panel after all, I have pushed an update that fixes this

[cdmichaelb]

Changes look good and nice documentation, thanks @MotherGinger