Added 2 new APIs (multiselect, multiremove)

Added 2 new APIs (multiselect, multiremove)
multiselect - Select multiple rows
multiremove - Remove multiple rows from existing selection

>>> multiselect('Open File', 'lstItemsView', ['Core.cs', 'Common.cs'])
1
>>> multiremove('Open File', 'lstItemsView', ['Core.cs', 'Common.cs'])
1

QA Notes:
Testing Done: Visual Studio File Open Dialog
Documentation Notes:
Bug Number:
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