NoTaint UIDropDownMenu

v7.2.0.24015 (2017/05/12)
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- added UIDropDownMenu.xml;
- removed customized codes in creating buttons;
- removed customized codes in Lib_UIDropDownMenu_AddButton()
- added LibStub to set library's version

v7.2.0.23801-r3 (2017/04/27)
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- fixed the loading order in ToC if this get installed separately.

v7.2.0.23801-r2 (2017/04/17)
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- ToC update to support proper loading if user install this library separately; this is for WoW 7.2 release.

v7.2.0.23801 (2017/03/22)
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- fixed the issue when dropdown menu has more than 1 level

v7.2.0.23789 (2017/03/22)
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- lib update to sync with WoW 7.2.0.23789
- TOC update to support WoW 7.2.0