Fix board selector synchronization

[AlbyIanna]

Motivation

The Board selector is not correctly synchronized when the board/port config changes, which results in inconsistent and confusing UI.

We also want to prevent deselecting the current board from the Board Selector by clicking on Unknown boards

#1204
#1206

Change description

• if no board is selected after closing the board selection dialog, restore the previously connected board https://github.com/arduino/arduino-ide/pull/1214/files#diff-85056a8e2611f77f68cc49f8ea30ab9e5e8ea370f0123e9f1ac63b404fc36195R247-R282
• rely on boardsServiceProvider when showing the currently selected board and port https://github.com/arduino/arduino-ide/pull/1214/files#diff-85056a8e2611f77f68cc49f8ea30ab9e5e8ea370f0123e9f1ac63b404fc36195L202-R213
• update available boards in boardsServiceProvider when fqbn changes (this fixes a bug that was there in RC8 too) https://github.com/arduino/arduino-ide/pull/1214/files#diff-e42c82bb67e277cfa4598239952afd65db44dba55dc7d68df619dfccfa648279R519

Other information

Reviewer checklist

• PR addresses a single concern.
• The PR has no duplicates (please search among the Pull Requests before creating one)
• PR title and description are properly filled.
• Docs have been added / updated (for bug fixes / features)

[per1234]

I verified this fixes:

• It's possible to unselect a board #1204
• Board Selector only updates on board selection via other interfaces if associated port is selected #1206

• update available boards in boardsServiceProvider when fqbn changes (this fixes a bug that was there in RC8 too)

Thanks Alberto!

[kittaakos]

I tried it and it worked reliably for me. Thank you for the fix!