Add warning to KeyAreaConverter c'tor when either loader or style parameter is invalid
Add warning to KeyAreaConverter c'tor when either loader or style parameter is invalid

RevBy: TrustMe
KeyAreaConverter would not complain about invalid parameter but simply crash
later. Now we at least print a critical warning before crashing. I decided to
not protect against the crashes, because it actually makes no sense: Being able
to transform the keyboards into key areas is a core feature, it simply *has* to
work. Better to make the surrounding code more robust.

maliit/plugins,KDE/plasma-maliit-plugins,RHawkeyed/plugins,RHawkeyed/plugins,maliit/plugins,RHawkeyed/plugins,RHawkeyed/plugins,develersrl/maliit-demo-qml,maliit/plugins,KDE/plasma-maliit-plugins,RHawkeyed/plugins,develersrl/maliit-demo-qml,KDE/plasma-maliit-plugins,develersrl/maliit-demo-qml,KDE/plasma-maliit-plugins,maliit/plugins,develersrl/maliit-demo-qml