Make TabWidget resizable

[JohannesLorenz]

In order to have variable sized control widgets in instrument windows for Lv2, we need its TabWidget (the one containing "Plugin", "Envelope, Filter & LFO", "Chord stacking & arpeggio" etc...) to be resizable. Currently it limits all its tabs to its own size and cuts them if required. In the end, it must have the maximum size of all tabs as minimum size.

There are two options:

• 👀 : add a boolean "fixedSize" to TabWidget's ctor
• 🚀 : Completely change TabWidget to always resize to fit its children.

Obviously, 🚀 will force us to fix a few more TabWidgets in LMMS. That will mean extra work, though it makes the TabWidget code less complex.

[he29-net]

You could implement the switch like @michaelgregorius described at #2847 (comment)
I already use his solution for the new spectrum analyzer, so implementing it for the TabWidget this way would make the "resizability switch" more consistent across other classes.

[JohannesLorenz]

I've implemented this with a boolean parameter. Will link the PR soon.

[JohannesLorenz]

Closing, as this is done in PR #4906