refactor!: Rename AppiumWebElement AppiumElement

[TroyWalshProf]

Change list

Rename AppiumWebElement AppiumElement

Types of changes

What types of changes are you proposing/introducing to .NET client?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• New feature (non-breaking change which adds value to the project)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Documentation

• Have you proposed a file change/ PR with appium to update documentation?

This can be done by navigating to the documentation section on http://appium.io selecting the appropriate command/endpoint and clicking the 'Edit this doc' link to update the C# example

Integration tests

• Have you provided integration tests to pass against the beta version of appium? (for Bugfix or New feature)

Details

The name AppiumWebElement can be confusing as it suggest all Appium elements are only web elements.
Things will be clearer is we use the name AppiumElement instead.

[TroyWalshProf]

Fixes #480
@akinsolb

[mykola-mokhnach]

I would rather implement this differently.
The old name should be marked as deprecated but PRESERVED and the new name should be added together with the old one, so users have some transition period to change their scripts.