Layer control Accessibility Bug Fixes

[Anshpreet8]

Fixed the following issues:

• shift the focus to first layer entry upon pressing enter to open the layer control
• shift focus to layer context menu when it's opened and shift focus back to the layer entry when it's closed

Closes #578

[prushforth]

LGTM - nice work

[Malvoz]

I noticed 2 things:

• If I shift + tab after the context menu has been opened focus always returns to the layer checkbox, it'd be good if focus instead returned to the button that was previously focused before opening the menu, for example the Layer Settings button.
• Pressing esc closes the context menu and the map is focused instead

[Anshpreet8]

@prushforth using the shadowRoot allowed me to get the current element in focus, I have accordingly made the suggested fixes

[prushforth]

Thanks, looks good!