Add some caching of IconManagers to combat issue #CLIENT-419 somewhat until we have a real fix.

With this code, starting up my test DMDirc (which loads a global window, and attempts to connect to a twitter:// server with a raw window) goes from creating to 30 IconManagers on startup and 4 every reconnect, to 8 on startup and none when reconnecting which is a start at least.

Change-Id: I9523a0165731bf294cb874ad9e088413d17882b8
Reviewed-on: http://gerrit.dmdirc.com/2972
Automatic-Compile: DMDirc Build Manager
Reviewed-by: Greg Holmes <greg@dmdirc.com>