While testing the original database from issue #1892 which contains
hundreds of tables and fields, I noticed how long the loading of the
database structure takes. This is especially problematic since it needs
to be reloaded on various occasions, e.g. running most statements in the
Execute SQL tab, which stalls the application every time.
According to a profiler it is the QIcon constructor which requires most
of the time. By introducing this small icon cache class we can reduce
the time for loading icons to almost nothing.
While still not perfect the UI already feels much more responsive with
this patch.
