's' hotkey in types view should be selection sensitive

[plafosse]

Version and Platform (required):

• Binary Ninja Version: 3.1 dev

Bug Description: In linearview when you make an inner selection of a structure the 's' hotkey says to make an anonymous structure of that size as a structure member. This same behavior should be replicated in types view.

[xusheng6]

In linear view, it does not make an anonymous structure -- it creates a structure with the next available name in the form of struct_n