Shows the global shortcuts in the capture mode ComboBox. These are the shortcuts that would perform a screenshot in that capture mode directly even if Spectacle wasn't currently open.
The purpose of this commit is to teach users these shortcuts, similar to how a QMenu does it, to boost users' speed and productivity long term.
Part of the credit for this MR goes to Leon De Andrade who uploaded a Phabricator revision to the same effect in 2019: https://phabricator.kde.org/D23446