Fix block analyzer framerate

Block analyzer uses an internal variable m_step to determine how
long to hold a bar up. This is dependant on framerate, however it is
only set on creation or resize of the analyzer. This patch changes
this value whenever the framerate is changed, preventing the analyzer
from appearing wildly fast or extremely slow until a restart.
Make framerate notification less nasty

Now uses a virtual function in Analyser::Base to notify any
analyser which whishes to listen
Merge pull request #4343 from TheUbuntuGuy/master

Fix block analyzer framerate