Rename `Text` to `TkText`

[pfitzseb]

and fix test errors on Windows and 0.6.

Fixes #117.

[pfitzseb]

Just noticed that this includes the fix from #136 -- not sure how we should handle that.

[pfitzseb]

OSX nightly test error is unrelated.

[aviks]

What was the motivation for changing Text to TkText? Much of the incidental changes in this PR is stale, but I'm trying to understand if the core change is still required.

[pfitzseb]

The fact that both Base and Tk export Text leads to fun stuff like

julia> Text("asd")
asd

julia> using Tk

julia> Text("asd")
ERROR: MethodError: no method matching Tk.TkWidget(::String, ::String)

julia> Base.Text("asd")
ERROR: MethodError: no method matching Tk.TkWidget(::String, ::String)

julia> Base.Docs.Text("asd")
ERROR: MethodError: no method matching Tk.TkWidget(::String, ::String)