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!
Excessive wordage.

"This method is used rather than a public virtual method to ensure that
ShutdownRequested is set to true"