* Layer Opacity: Fix selection changes would not update while window was open

The reason is that we didn't emit selection changes while moving, which meant that the opacity window would only have the fade in/out option available if you had frames selected before opening it.

* Make none of the buttons in the dialog default enabled

* Fix layer opacity fade in/out would not fade entirely

When using fade in with initial opacity on 100%
The result would look like:
1. 33%
2. 66%
3. 100%

However I expected:
1. 0%
2. 50%
3. 100%

Similar when using fade out with initial opacity of 100%
1. 100%
2. 66%
3. 33%

I expected
1. 100%
2. 50%
3. 0%

* Fix case where if the last frame was null, framesModified wouldn't be called

Even though we might have updated other keyframes.

* Apply small simplification

- Ref: actions/checkout#1809 (comment)

100% reviewed source file: 'app/translations/mui.pot'
on 'cs'.

Co-authored-by: transifex-integration[bot] <43880903+transifex-integration[bot]@users.noreply.github.com>