Make MnemonicAttached active property writable
MnemonicAttached is used in Plasma applets (in particular in AppMenu applet) which can't use eventFilter to handle Alt key change. Instead this commit adds an ability to show underlined rich text by setting active property programmatically. Event filter will be removed when active is set so anyone can use it to force show mnemonics from code.
Needed for plasma/plasma-workspace#16 (closed)