Feature/issue122/back button menu

[annapiktas]

What's in this PR?

It looks like Back Button menu - UI context menu is already supported. The elements/items of the UI context menu conform to the Button and can be accessed by the title/ name therefore there is no need to create a new element for UI context menu nor for the elements.

Worth mentioning that the UI context menu elements/ items are available on the page even when UI context menu is not revealed/ opened. However there seems to be an issue on UI when long press reveals the UI context menu but makes the elements not accessible. This seems to be UI issue only that might need to be investigated later, possibly as separate issue.

Adde in this PR:

• An extra page after TableSelectionScreen to get a UI context menu with two items/elements
• A simple test for usage of UI context menu

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Pre-merge checklist

Before merging any PR, please check the following common things that should be done beforehand. These aren't all always required, so just check the box if it doesn't apply.

• When adding files, make sure they're added to the right target. If you're adding new files that should be bundled up with Cocoapods etc, they need to be added to the TABTestKit target, not Pods-TABTestKit_Example etc.
• Run pod install to ensure that the latest changes are in the Example project. Without this, Carthage might not see the latest changes.
• Added and updated tests where possible. This isn't always possible but try wherever you can. The example app contains UI tests to test many of the TABTestKit features.
• Updated the CHANGELOG. For any changes pending a release, add to the Pending section. For releases, move everything pending to the release section.
• Updated the README. Add info for any new features, update existing info for anything that's changed or needs extra info.