Unification of Dynamo shutdown sequence

# Background
This pull request is meant to unify the way ```DynamoModel``` is shutdown (more specifically, ```RevitDynamoModel```). During the shutdown process, ```DynamoModel.ShutDown``` takes charge of the sequence in which virtual methods like ```PreShutdownCore```, ```ShutdownCore``` and ```PostShutdownCore``` are called. ```DynamoModel.ShutDown``` method also raises events like ```ShutdownStarted``` and ```ShutdownCompleted``` at very specific points in the shutdown process, allowing more deterministic event handling on higher layers like ```DynamoViewModel```.

# Changes
### Existing shutdown behavior
##### Shutdown sequence 1 (an ideal scenario):

    1. DynamoView.WindowClosing > RevitDynamoModel.ShutDown
    2. RevitDynamoModel.ShutDown > IdlePromise.ExecuteOnShutdown (delete Keeper ElementId)
    3. RevitDynamoModel.ShutDown > DynamoModel.ShutDown
    4. DynamoModel.ShutDown
    5. RevitDynamoModel.ShutDown > IdlePromise.ExecuteOnIdleAsync (PostableCommand.ExitRevit)
    6. DynamoRevit.DynamoView_Closing > IdlePromise.ClearPromises
    7. DynamoRevit.DynamoView_Closing > IdlePromise.Shutdown
    8. DynamoRevit.DynamoView_Closed
    9. Shutdown delegate (from Step #2 above)

##### Shutdown sequence 2 (incorrect scenario):

    1. DynamoView.WindowClosing > RevitDynamoModel.ShutDown
    2. DynamoRevit.DynamoView_Closing > IdlePromise.ClearPromises
    3. DynamoRevit.DynamoView_Closing > IdlePromise.Shutdown
    4. DynamoRevit.DynamoView_Closed
    5. RevitDynamoModel.ShutDown > IdlePromise.ExecuteOnShutdown (delete Keeper ElementId)
    6. RevitDynamoModel.ShutDown > DynamoModel.ShutDown
    7. DynamoModel.ShutDown
    8. RevitDynamoModel.ShutDown > IdlePromise.ExecuteOnIdleAsync (PostableCommand.ExitRevit)
    9. Shutdown delegate NOT executed (#5 comes after #3)

There are various ways a shutdown sequence can be initiated:

1. Pressing ```X``` button on Dynamo window (sequence 1)
2. By update manager (sequence 2)
3. File > Exit menu item (sequence 2)

### Unified shutdown process
The new shutdown process starts with a call to ```DynamoViewModel.PerformShutdownSequence```. Regardless of the caller (i.e. the ```Shutdown Triggers```), it performs the shutdown in the following order:

    Shutdown Triggers > DynamoViewModel.PerformShutdownSequence
      PerformShutdownSequence > DynamoModel.ShutDown
        DynamoModel.ShutDown > OnShutdownStarted
          OnShutdownStarted > IdlePromise.ExecuteOnShutdown (delete Keeper ElementId)
        DynamoModel.ShutDown > RevitDynamoModel.PreShutdownCore
          RevitDynamoModel.PreShutdownCore > IdlePromise.ExecuteOnShutdown (ExitRevit command)
        DynamoModel.ShutDown > RevitDynamoModel.ShutdownCore
          RevitDynamoModel.ShutdownCore > DynamoModel.ShutdownCore
        DynamoModel.ShutDown > RevitDynamoModel.PostShutdownCore
          RevitDynamoModel.ShutDown > IdlePromise.ClearPromises
          RevitDynamoModel.ShutDown > IdlePromise.Shutdown
        DynamoModel.ShutDown > OnShutdownCompleted
    DynamoRevit.DynamoView_Closed
    Shutdown delegates (from 'ExecuteOnShutdown' calls above)

Interesting facts about this shutdown process:

1. If the shutdown process should also close Revit, ```PostableCommand.ExitRevit``` will be sent through ```IdlePromise.ExecuteOnShutdown``` (instead of ```ExecuteOnIdleAsync```), ensuring that it does not get cleared by ```IdlePromise.ClearPromises```.
2. ```DynamoModel.ShutdownStarted``` event is added and called right before ```PreShutdownCore```.
3. ```DynamoModel.ShutdownCompleted``` event is added and called right after ```PostShutdownCore```.

# Testing scenarios
1. Tested both shutdown delegates are executed regardless of the ```Shutdown Triggers```.
2. Tested the following scenarios for both ```Revit 2014``` and ```Revit 2015```:
 - Pressing ```X``` button on Dynamo window
 - Pressing ```CTRL + F4``` shortcut key
 - Fake update manager call to install update
 - ```File > Exit``` menu item
3. Tested preferences (e.g. window position/size) are remembered with all above shutdown triggers.
4. Ensured both ```CLEAN_SHUTDOWN_VALUE``` and ```CRASHING_SHUTDOWN_VALUE``` continue to be tracked properly by ```StabilityTracking```.

# Notes to reviewer
Hi @aparajit-pratap, please have a go with this. I have placed Revit+Dynamo under the debugger for all the above scenarios, the program flows as expected. Please see if you can spot something outside of that natural flow. Thanks!
Consider " 'shutdownSequenceInitiated' is marked as true here indicating that the shutdown may not be stopped"