Some fixes related to clipboard (#2405)

Also:
- Fixed capitalization of one method in `LoginDialog.cpp`
- Visually improved some strings in `LoginDialog.cpp`
- Added utility function getting clipboard content to `Clipboard.cpp`
- Made sure to use `crossPlatformCopy` in `UserInfoPopup.cpp`

src/util/Clipboard.cpp

@@ -6,11 +6,18 @@ namespace chatterino {
 void crossPlatformCopy(const QString &text)
 {
     auto clipboard = QApplication::clipboard();
+
     clipboard->setText(text);
+
     if (clipboard->supportsSelection())
     {
         clipboard->setText(text, QClipboard::Selection);
     }
 }
 
+QString getClipboardText()
+{
+    return QApplication::clipboard()->text();
+}
+
 }  // namespace chatterino