Reviewed-by: Thorbjørn

While before you had to call add, then remove for each context id and
then call update, now you call updateAdditionalContexts with a list of
contexts to remove and add. It has the update step built in.

Reviewed-by: con

the previous implementation relied on poking around with setVisible
of the menu actions. Now, command attribute CA_Hide is used instead for
the necessary entries.

Reviewed-by: hjk

Additionally, hiding of debugger menu entries was done as most of
them are irrelevant for QML debugging.

The new QML Inspector depends on DebuggerPlugin. Also added a dropdown menu into
the debugger toolbar from which the user can select the used debugging
language, e.g. C++ or QML.