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.hpp

@@ -6,4 +6,6 @@ namespace chatterino {
 
 void crossPlatformCopy(const QString &text);
 
+QString getClipboardText();
+
 }  // namespace chatterino