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()`

samplebrain.pro

@@ -10,8 +10,9 @@ INCLUDEPATH += . 2
 QT += core gui widgets
 
 # Input
-HEADERS += app/MainWindow.h \
-           app/generated/ui_samplebrain.h \
+HEADERS += app/MainWindow.h
+
+FORMS += gui/samplebrain.ui
 
 SOURCES += app/MainWindow.cpp \
            app/sound_items.cpp \