Fix a few memory leaks

ASAN: Fix leak in XWindowTasksModel

Summary:
Trace:
Direct leak of 24 byte(s) in 3 object(s) allocated from:
    #0 0x544cc0 in operator new(unsigned long) (/home/kfunk/devel/install/kf5/bin/plasmashell+0x544cc0)
    #1 0x7fe4f0e82df9 in TaskManager::XWindowTasksModel::Private::windowInfo(unsigned long long) /home/kfunk/devel/src/kf5/plasma-workspace/libtaskmanager/xwindowtasksmodel.cpp:419:25
    #2 0x7fe4f0e87e69 in TaskManager::XWindowTasksModel::data(QModelIndex const&, int) const /home/kfunk/devel/src/kf5/plasma-workspace/libtaskmanager/xwindowtasksmodel.cpp:646:38
    ...

ASAN: Fix leak in ShellCorona

Trace:
Indirect leak of 48 byte(s) in 1 object(s) allocated from:
    #0 0x544cc0 in operator new(unsigned long) (/home/kfunk/devel/install/kf5/bin/plasmashell+0x544cc0)
    #1 0x5e1a95 in ShellCorona::checkAddPanelAction(QStringList const&) /home/kfunk/devel/src/kf5/plasma-workspace/shell/shellcorona.cpp:1728:27
    #2 0x5dac7e in ShellCorona::ShellCorona(QObject*) /home/kfun...