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/common/ChatterSet.hpp

@@ -9,6 +9,7 @@
 #include <set>
 #include <unordered_map>
 #include <unordered_set>
+#include <vector>
 
 namespace chatterino {
 

src/common/enums/MessageOverflow.hpp

@@ -0,0 +1,18 @@
+#pragma once
+
+namespace chatterino {
+
+// 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,
+};
+
+}  // namespace chatterino