Move all platform specific code to platform's folders.

[afedchin]

Description

Move all platform specific code which implements generic functionality into a platform folder.

Motivation and Context

Currently we have a lot of specific platform's code spread-ed to the whole repository. This is a try to keep all platform specific code in one place.
As an example Win32 platform had the following folders structure:

/xbmc
  |- filesystem
  |   |- win32
  |- network
  |   |- win32
  |- peripherals
  |   |- bus
  |       |- win32
  |- powermanagement
  |   |- win32
  |- storage
  |   |- win32
  |- utils
      |- win32

Now it has:

/xbmc/platform/win32
  |- filesystem
  |- network
  |- peripherals
  |- powermanagement
  |- storage
  |- utils

This doesn't touch threads for now to ensure that do not produce conflicts with @jimfcarroll's PR #13721

How Has This Been Tested?

Don't run-time tested yet. As because this doesn't change business logic I think green light at Jenkins is enough.

Screenshots (if appropriate):

Types of change

• Bug fix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves existing functionality)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the Code guidelines of this project
• My change requires a change to the documentation, either Doxygen or wiki
• I have updated the documentation accordingly
• I have read the CONTRIBUTING document
• I have added tests to cover my change
• All new and existing tests passed

[FernetMenta]

great!

[Memphiz]

Nice work - thx for this digging work :)

[popcornmix]

Builds and runs on Pi, so no obvious problem here.

[Rechi]

looks good
found some win10 changes not related to this PR title, but it's fine for me if you want to keep them in this PR.

[afedchin]

thanks for reviewing.