Maximize button for resizable instruments

[michaelgregorius]

Show the maximize button for resizable instruments.

Most other changes have the character of refactorings and code reorganizations.

Remove the negation in the if condition for resizable instruments to make the code better readable.

Only manipulate the system menu if the instrument is not resizable.

Add a TODO to the special code that sets a size.

Unfortunately there are still some artifacts for some reason, i.e. the title bar still seems to be shown for a maximized instrument window:

[michaelgregorius]

Ok, this one is weird. I have added the following code snippet to MainWindow::finalize right after the for loop that adds the editor windows:

auto wid = new QWidget();
wid->setMinimumSize(QSize(200, 100));
wid->setWindowTitle("Window title");
//wid->setStyleSheet("background-color:#108010;");
addWindowedWidget(wid);

As you can see setting the style sheet is commented out and can be commented back in. Here's how the maximized window looks without a style sheet being set:

The title bar of the window can still be seen for some reason.

Here's the same situation but with the style sheet being set for the new window/widget:

Now there is no title bar and it looks like expected.

I guess the instrument window is in some strange in between state.

Does anybody have an idea what causes this behavior?

[michaelgregorius]

This seems to be caused by LMMS' own SubWindow class because it works as expected if I directly add a QMdiSubWindow via the MDI area:

auto wid = new QWidget();
wid->setMinimumSize(QSize(200, 100));
wid->setWindowTitle("Window title");
m_workspace->addSubWindow(wid);

[michaelgregorius]

Maximizing also works as expected when the instrument window is added as a "normal" QMdiSubWindow, i.e. not as a SubWindow:

[michaelgregorius]

This PR is now ready for review as I have managed to fix the problems with the SubWindow class so that it now looks like in the screenshot above:

I have also added some other small improvements to the SubWindow class.

[sakertooth]

@michaelgregorius, I have plans to revert a good chunk of #7453, which would affect this PR. I don't think we should keep moving forward with making SlicerT bigger/resizable for now until we properly rework the instrument window to support larger sizes the right way. Effect racks and (at the very least) Lv2 layouts do not render properly and the envelope tab is massively distorted.

We will always have another chance at making the instrument windows bigger, it's not like I am throwing that idea out completely. I just don't think its worth progressing further with this knowing the massive regressions we still have to deal with (and I do not like how resizability/bigger sizes for instruments was implemented).