When switching LVS_NOCOLUMNHEADER on/off change the style of the header control HDS_HIDDEN.

The native control issues the CreateWindowEx call with the HDS_HIDDEN
as part of the style and not by issuing the SetWindowLong call.

ntdll/tests: Add tests for (Rtl)IsTextUnicode to verify that IS_TEXT_UNICODE_CONTROLS and IS_TEXT_UNICODE_REVERSE_CONTROLS are not mutually exclusive.

Tests for undo coalescing from previous patches sent as suggested by
Alex Villacís Lasso.

The bCaretAtEnd value in ME_TextEditor is used to identify that the
caret is at the end of a wrapped line instead of the start of the next
line in the paragraph since both these positions correspond to the
same position in the document. The bCaretAtEnd value was previously
being set back to FALSE whenever the window was resized.