Add a Send button that can be used to send messages (#4607)

This button is disabled by default, and can be enabled with the "Show send message button" setting.

src/widgets/splits/SplitInput.hpp

@@ -23,19 +23,6 @@ class MessageThread;
 class ResizingTextEdit;
 class ChannelView;
 
-// MessageOverflow is used for controlling how to guide the user into not
-// sending a message that will be discarded by Twitch
-enum MessageOverflow {
-    // Allow overflowing characters to be inserted into the input box, but highlight them in red
-    Highlight,
-
-    // Prevent more characters from being inserted into the input box
-    Prevent,
-
-    // Do nothing
-    Allow,
-};
-
 class SplitInput : public BaseWidget
 {
     Q_OBJECT
@@ -132,6 +119,7 @@ protected:
     struct {
         ResizingTextEdit *textEdit;
         QLabel *textEditLength;
+        EffectLabel *sendButton;
         EffectLabel *emoteButton;
 
         QHBoxLayout *hbox;