Add more ignored paths for AppVeyor CI

AppVeyor CI builds for Microsoft Windows only. Therefore, do not start builds
for changes made only in dfb, gtk, gtk1, motif, unix, and x11 folders.

Additionally, do not start the builds when only an *.md file in the root
folder changes.

Lastly, ignore all changes in the demos folder: Demos are not built by AppVeyor at all.

appveyor.yml

@@ -7,11 +7,25 @@ branches:
 
 skip_commits:
   files:
+    - demos/
     - docs/
     - interface/
     - misc/
+    - include/wx/dfb/
+    - src/dfb/
+    - include/wx/gtk/
+    - src/gtk/
+    - include/wx/gtk1/
+    - src/gtk1/
+    - include/wx/motif/
+    - src/motif/
     - include/wx/osx/
     - src/osx/
+    - include/wx/unix/
+    - src/unix/
+    - include/wx/x11/
+    - src/x11/
+    - '*.md'
     - .github/workflows/
     - .travis.yml
     - build/tools/travis-ci.sh