Originally this used both SolarMutex and an own mutex, then a deadlock
was resolved in 2fe6a4a3 by not locking
SolarMutex.

Since the class will call event listeners without dropping the mutex
e.g. in insertNoCheck(), using the SolarMutex appears better anyway.

With this, installing a BASIC extension no longer triggers SolarMutex
asserts in SfxBroadcaster.

Change-Id: Ib9a2ee491ef53b1a53855af0fc22e863c5e7cb91