Update project to Qt6:

* loaded and saved `samplebrain.ui` in Qt6 Designer
* generate `ui_samplebrain.h` on the fly (via `FORMS +=`)
* remove example files `sample.ui` and `ui_sample.h`
* remove `setMargin()` calls, these methods have been removed
* use `QFont::Bold` for `setWeight()` calls, numeric values aren't accepted anymore
* qualify `spiralcore::sample` in `block.cpp`'s `normalise()`

app/MainWindow.h

@@ -18,7 +18,7 @@
 #include <QDirIterator>
 #include <QFileDialog>
 #include <QLineEdit>
-#include "generated/ui_samplebrain.h"
+#include "ui_samplebrain.h"
 
 #include <iostream>
 #include <lo/lo.h>