Bump gocui

... and switch back from stefanhaller's tcell fork to the official tcell. This
basically reverts 7ccb871a45.

vendor/github.com/gdamore/tcell/v2/attr.go

@@ -0,0 +1,33 @@
+// Copyright 2020 The TCell Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use file except in compliance with the License.
+// You may obtain a copy of the license at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package tcell
+
+// AttrMask represents a mask of text attributes, apart from color.
+// Note that support for attributes may vary widely across terminals.
+type AttrMask int
+
+// Attributes are not colors, but affect the display of text.  They can
+// be combined.
+const (
+	AttrBold AttrMask = 1 << iota
+	AttrBlink
+	AttrReverse
+	AttrUnderline
+	AttrDim
+	AttrItalic
+	AttrStrikeThrough
+	AttrInvalid              // Mark the style or attributes invalid
+	AttrNone    AttrMask = 0 // Just normal text.
+)