Add error pop-up message for missing description

If a description file is missing, the video will pause and notify the
user.
Add MessageBoxFactory class

Adds a MessageBox generator to simplify code
Add ICommand Extensions

Add parameter-less method calls to reduce noise.
Update PropertyChanged invokers

Use the CallerMemberName attribute instead of a magic string.
Merge branch 'valdev' of https://github.com/vserrago/LiveDescribe-Desktop into danieldev

Conflicts:
	LiveDescribe/LiveDescribe/Model/Description.cs
	LiveDescribe/LiveDescribe/View Model/MainWindowViewModel.cs