eui: Add TextBoxEx header

[guymakinggames]

This change is 

[MonsterDruide1]

@MonsterDruide1 reviewed 1 file and all commit messages, and made 2 comments.
Reviewable status: all files reviewed, 2 unresolved discussions (waiting on guymakinggames).

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include/euiTextBoxEx.h line 11 at r1 (raw file):

    u8 _0[0x11c];
    u8 mDirtyFlags;
};

Suggestion:

class TextBoxEx : public nn::ui2d::TextBox {
public:
    // will change accordingly
    u8 _0[0x11c];
    u8 mDirtyFlags;
};

---

include/euiTextBoxEx.h line 20 at r1 (raw file):

template <>
eui::TextBoxEx* DynamicCast<eui::TextBoxEx*, nn::ui2d::Pane>(nn::ui2d::Pane* pane);
}  // namespace nn::ui2d

You got a jackpot of "missing dependencies":
This one needs a PR to nnheaders as well, as nn::ui2d should be listed in that repo instead.
https://github.com/open-ead/nnheaders

Suggestion:

namespace nn::ui2d {
template <typename T, typename U>
T DynamicCast(U* pane);
}  // namespace nn::ui2d