Make Emote Tab Completion Search for Substrings (#1204)

* Make emote tab completion search for substrings

Previously, tab completion only searched for emotes that start
with the current prefix under the cursor. This commit makes tab
completion look for the current prefix as a substring in any position,
not just the start.

Examples:
    * In forsen's channel, tabbing on "pls" will cycle through "DonaldPls", "forsenPls",
    "GachiPls", "nyanPls" and "SourPls".

    * As a forsen subscriber, tabbing on "sen1" will complete to "forsen1".

    * As a pajlada subscriber, tabbing on "shrug" will cycle through
    "pajaShrugL" and "pajaShrugR". (Unless you are in a channel with
    more "shrug" emotes, of course.)

* Add a setting for prefix and substring completion

This commit adds a setting under the "Miscellaneous" section to allow
user to choose whether they want prefix-only or substring emote
completion.

The QCompleter filter mode and `addString` function are now chosen
according to the user's setting.

* Improve description of emote completion setting

Also reintroduce a blank line that went missing.

src/common/CompletionModel.cpp

@@ -10,6 +10,7 @@
 #include "providers/twitch/TwitchChannel.hpp"
 #include "providers/twitch/TwitchServer.hpp"
 #include "singletons/Emotes.hpp"
+#include "singletons/Settings.hpp"
 
 #include <QtAlgorithms>
 #include <utility>
@@ -78,17 +79,28 @@ int CompletionModel::rowCount(const QModelIndex &) const
 
 void CompletionModel::refresh(const QString &prefix)
 {
+    std::function<void(const QString &, TaggedString::Type)> addString;
+    if (getSettings()->prefixOnlyEmoteCompletion)
+    {
+        addString = [&](const QString &str, TaggedString::Type type) {
+            if (str.startsWith(prefix, Qt::CaseInsensitive))
+                this->items_.emplace(str + " ", type);
+        };
+    }
+    else
+    {
+        addString = [&](const QString &str, TaggedString::Type type) {
+            if (str.contains(prefix, Qt::CaseInsensitive))
+                this->items_.emplace(str + " ", type);
+        };
+    }
+
     std::lock_guard<std::mutex> guard(this->itemsMutex_);
     this->items_.clear();
 
     if (prefix.length() < 2)
         return;
 
-    auto addString = [&](const QString &str, TaggedString::Type type) {
-        if (str.startsWith(prefix, Qt::CaseInsensitive))
-            this->items_.emplace(str + " ", type);
-    };
-
     if (auto channel = dynamic_cast<TwitchChannel *>(&this->channel_))
     {
         // account emotes