Added GitHub actions OS X build

.github/workflows/osx.yml

@@ -0,0 +1,21 @@
+name: OS X
+
+on:
+  push:
+    branches:
+      - master
+
+jobs:
+  build:
+    name: GPXSee
+    runs-on: macos-10.15
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Install dependencies
+        run: brew install qt5
+      - name: Configure build
+        run: lrelease gpxsee.pro
+        run: qmake gpxsee.pro
+      - name: Build project
+        run: make -j3