qt: use implicit content size for defining control implicit size

Qt 6.9 starts [1] to use implicit content size for the control implicit size, which should be more appropriate. Whether intentional or not, it appears that we already realized this as most cases in our code we already use implicit content size for defining the control implicit size.

The boilerplate implicit size code for controls are inherited from a style (I usually copy from Fusion). We have to do this everytime we directly derive from a template type because we (still) do not have our own style that sets this.

I have tried to check this if this change affects the mini player (where we use the implicit size), but could not find an issue. Please also check yourself.

Request review @chub.

[1] https://codereview.qt-project.org/c/qt/qtdeclarative/+/610741