Add macOS and Windows CI workflows, remove old disabled ones

Delete the three old qt-*.yml workflows (disabled, Qt 5.12) and
replace with new macos-build.yml and windows-build.yml using Qt 6.
All three platforms now build on push/PR to dev and master branches.

Author: aalex

.github/workflows/macos-build.yml

@@ -0,0 +1,30 @@
+# Build MapMap on macOS with Qt 6
+name: Build on macOS
+
+on:
+  push:
+    branches: [ dev, master ]
+  pull_request:
+    branches: [ dev, master ]
+
+jobs:
+  build:
+    runs-on: macos-latest
+
+    steps:
+    - name: Checkout code
+      uses: actions/checkout@v4
+
+    - name: Install dependencies
+      run: |
+        brew install qt liblo
+
+    - name: Configure with qmake
+      run: |
+        export PATH="$(brew --prefix qt)/bin:$PATH"
+        qmake mapmap.pro CONFIG+=release
+
+    - name: Build
+      run: |
+        export PATH="$(brew --prefix qt)/bin:$PATH"
+        make -j$(sysctl -n hw.logicalcpu)

.github/workflows/qt-linux.yml

@@ -0,0 +1,31 @@
+# Build MapMap on Windows with Qt 6
+name: Build on Windows
+
+on:
+  push:
+    branches: [ dev, master ]
+  pull_request:
+    branches: [ dev, master ]
+
+jobs:
+  build:
+    runs-on: windows-latest
+
+    steps:
+    - name: Checkout code
+      uses: actions/checkout@v4
+
+    - name: Install Qt 6
+      uses: jurplel/install-qt-action@v4
+      with:
+        version: '6.8.0'
+        host: 'windows'
+        target: 'desktop'
+        arch: 'win64_mingw'
+        modules: 'qtmultimedia'
+
+    - name: Configure with qmake
+      run: qmake mapmap.pro CONFIG+=release
+
+    - name: Build
+      run: mingw32-make -j4