Add formspec interactions for Player

[S-S-X]

Basic functionality:

• Assertion that verifies received fomrspec.
  • At any point verify that player received any formspec.
    • Would be good to also allow verifying that formspec was not sent, invert error condition.
    • ✔ Resolution: assert.is_Form & assert.not_Form.
  • Optional callback for closer formspec inspection.
    • Possible assertions here should be included in callback function, no automation
    • ✔ Resolution: no callbacks, instead Form class implements facilities to find or iterate elements and read element properties.
• Submit formspec with simple actions.
  • Track and send default values recevied with formspec.
    • ✔ Resolution: Form keeps elements and original data separated.
  • Simple button interactions, hit named button.
  • Close formspec (when player hits escape key).
    • Possible resolution: just use core.close_formspec(playername, formname).
  • Send formspec events (selections, scroll and so on).

Keep track of:

• How many formspec updates have been received.
• Possibly history of formspec names and getting count based on formspec name.
• Validate that player is online if server module is active.

Preview (included in latest docker release):

• Based on v0.15 work v0.15 #105
• Update: moved to Engine version compat 0.15 #118