refactor: Make Account & Settings buttons SVGs (#6267)

instead of rendering the icons directly in Chatterino, they're now SVGs

This has the following consequences:
1. The color does _not_ respect your custom theme. For most users, this
   will not be a problem/concern.
2. You can no longer drag a split onto those buttons to create a new tab

src/widgets/Notebook.cpp

@@ -13,6 +13,7 @@
 #include "widgets/buttons/InitUpdateButton.hpp"
 #include "widgets/buttons/NotebookButton.hpp"
 #include "widgets/buttons/PixmapButton.hpp"
+#include "widgets/buttons/SvgButton.hpp"
 #include "widgets/dialogs/SettingsDialog.hpp"
 #include "widgets/helper/ChannelView.hpp"
 #include "widgets/helper/NotebookTab.hpp"
@@ -1407,7 +1408,12 @@ void SplitNotebook::showEvent(QShowEvent * /*event*/)
 void SplitNotebook::addCustomButtons()
 {
     // settings
-    auto *settingsBtn = this->addCustomButton<NotebookButton>();
+    auto *settingsBtn = this->addCustomButton<SvgButton>(SvgButton::Src{
+        .dark = ":/buttons/settings-darkMode.svg",
+        .light = ":/buttons/settings-lightMode.svg",
+    });
+
+    settingsBtn->setPadding({0, 0});
 
     // This is to ensure you can't lock yourself out of the settings
     if (getApp()->getArgs().safeMode)
@@ -1426,14 +1432,18 @@ void SplitNotebook::addCustomButtons()
             this->signalHolder_);
     }
 
-    settingsBtn->setIcon(NotebookButton::Settings);
-
-    QObject::connect(settingsBtn, &NotebookButton::leftClicked, [this] {
+    QObject::connect(settingsBtn, &Button::leftClicked, [this] {
         getApp()->getWindows()->showSettingsDialog(this);
     });
 
     // account
-    auto *userBtn = this->addCustomButton<NotebookButton>();
+    auto *userBtn = this->addCustomButton<SvgButton>(SvgButton::Src{
+        .dark = ":/buttons/account-darkMode.svg",
+        .light = ":/buttons/account-lightMode.svg",
+    });
+
+    userBtn->setPadding({0, 0});
+
     userBtn->setVisible(!getSettings()->hideUserButton.getValue());
     getSettings()->hideUserButton.connect(
         [userBtn](bool hide, auto) {
@@ -1441,8 +1451,7 @@ void SplitNotebook::addCustomButtons()
         },
         this->signalHolder_);
 
-    userBtn->setIcon(NotebookButton::User);
-    QObject::connect(userBtn, &NotebookButton::leftClicked, [this, userBtn] {
+    QObject::connect(userBtn, &Button::leftClicked, [this, userBtn] {
         getApp()->getWindows()->showAccountSelectPopup(
             this->mapToGlobal(userBtn->rect().bottomRight()));
     });