25697 - Updates to improve UI behaviour in 3ds Max

Updates to improve UI behaviour in 3ds Max.  These are general improvements but were specifically a problem when implementing the Perforce integration.
- Added a pre-app init method to the Engine base class.  This allows any initialisation to be done after Qt, etc. have been set up but before the apps are loaded - this is particularly useful if any additional setup is needed before any 'launch-on-init' apps are loaded.
- Tweaked the Toolkit dialog layout so that it doesn't enforce an overly large (tall) minimum size which resulted in oddly sized dialogs, particularly when they were supposed to be quite short (e.g. the Perforce password entry dialog)