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()`

brain/src/block.cpp

@@ -36,7 +36,7 @@ double square(double a) {
   return a*a;
 }
 
-void normalise(sample &in) {
+void normalise(spiralcore::sample &in) {
   // find min/max
   float max = 0;
   float min = FLT_MAX;