Made FadingMessageWindow implement all UI logic on the UI thread

[tombogle]

This fixes crashes like SP-2340
I have marked this as a breaking change because of the removal of protected members, but I know of no derived classes, and in practice, the existing implementation was such that these members would be unlikely to have been meaningful or helpful in a derived class anyway.

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This change is 

[github-actions]

Palaso Tests

     4 files  ±0       4 suites  ±0   17m 4s ⏱️ -18s
 4 880 tests ±0   4 652 ✅ ±0  228 💤 ±0  0 ❌ ±0 
15 890 runs  ±0  15 206 ✅ ±0  684 💤 ±0  0 ❌ ±0 

Results for commit 1f7f59a. ± Comparison against base commit a4215d3.

♻️ This comment has been updated with latest results.

[hahn-kev]

works for me, though the popup message was not relative to the test app form which was surprising.

[tombogle]

Yeah, I probably could have (and maybe should have) gotten it to pop up in a more sensible place, but for the test app I didn't care. I just wanted to make sure it appeared and faded as expected.

[tombogle]

Reviewable status: 0 of 6 files reviewed, 2 unresolved discussions (waiting on @hahn-kev and @josephmyers)