Add platform API for native inputs (dropdowns, file selection, calendar, etc.)

[adamjs]

Add an interface for users to handle modal display and handling of native input menus (eg, <select> and <input type='file'>.

Should add default implementation for each platform to AppCore.

[adamjs]

Related: #70

[joepriit]

This could possibly be used for implementation:
https://github.com/native-toolkit/tinyfiledialogs

The only problem seems to be with selecting multiple files (50+).

[raphael10-collab]

@adamjs Is already implemented? I tried standard HTML5 input capabilities but none work:

https://www.w3schools.com/tags/tryit.asp?filename=tryhtml5_input_type_file