Added Theme support for Treeview control

[fabiofranzini]

Q	A
Bug fix?	[ ]
New feature?	[X]
New sample?	[ ]
Related issues?	fixes #1196

What's in this Pull Request?

With this PR I resolved the problem described in #1196.

I made these changes:

• added to the theme property to the control
• changed the CSS file using CSS variables
• changed the TreeView main control to inject CSS variables for theming and child controls to add the Theme object as property
• changed the documentation
• updated the test Web Part.

In preferred to not rewrite all the control (because it's a complex control) and maintain the original CSS file by using CSS variable to inject the correct colors.

This is the result:

[AJIXuMuK]

Thank you @fabiofranzini!
Please, next time do not shuffle the import declarations. This complicates review process.

[fabiofranzini]

Thank you @fabiofranzini! Please, next time do not shuffle the import declarations. This complicates review process.

Yes, sorry for that!
I thought about it when I did it, I'm used to always "organize using".