Issue audacity#2226: Joining multiple clips sometimes time-shifts them

Steps to reproduce:

1. Split an audio clip into three clips
2. Move the clips so that there are spaces between the clips.
3. Select all the audio (Ctrl+A)
4. Join the clips (Ctrl+J)
5. Observe that clips are not joined correctly.

Problem:
In  WaveClip::InsertSilence(), the following two lines:

else if (t == GetPlayEndTime() && t < GetSequenceEndTime())
      ClearSequence(t, GetSequenceEndTime());

After ClearSequence is called to remove the data to the end of the sequence, then there is not call to update mTrimRight, as is done in WaveClip::ClearRight(). So when more data is pasted, it's pasted in the wrong place.

Fix:
Update mTrimeRight.

Notes:
1. I don't know why the bug was not always reproduceable.
2. There appears to be a similar issue with the previous two lines:

   if (t == GetPlayStartTime() && t > GetSequenceStartTime())
      ClearSequence(GetSequenceStartTime(), t);

Compare with WaveClip::ClearLeft().
However I have left that unfixed, both because it doesn't affect this bug, and also because I couldn't work out when that call to ClearSequence would get called, and so I couldn't test any fix.

src/WaveClip.cpp

@@ -1382,8 +1382,10 @@ void WaveClip::InsertSilence( double t, double len, double *pEnvelopeValue )
 {
    if (t == GetPlayStartTime() && t > GetSequenceStartTime())
       ClearSequence(GetSequenceStartTime(), t);
-   else if (t == GetPlayEndTime() && t < GetSequenceEndTime())
+   else if (t == GetPlayEndTime() && t < GetSequenceEndTime()) {
       ClearSequence(t, GetSequenceEndTime());
+      SetTrimRight(.0);
+   }
 
    auto s0 = TimeToSequenceSamples(t);
    auto slen = (sampleCount)floor(len * mRate + 0.5);