Implement bootstrap-gstreamer for all platforms

This change makes it so that the Platform classes can now handle
installing GStreamer dependencies and properly setting up the
environment including when cross-compiling. For Windows and Linux
is now installed into `target/dependencies/gstreamer` when not installed
system-wide. In addition:

1. Creating and moving existing environment path append helpers to
   `util.py`.
2. Combining the `set_run_env` and `build_dev` functions and moving
   some outside code into them so that it can be shared. Now code that
   used to call `set_run_env` calls `build_dev` and then
   `os.environ.update(...)`.
3. Adding Python typing information in many places.

Signed-off-by: Martin Robinson <mrobinson@igalia.com>

.github/workflows/mac.yml

@@ -66,7 +66,7 @@ jobs:
       - name: Bootstrap
         run: |
           python3 -m pip install --upgrade pip virtualenv
-          bash etc/install_macos_gstreamer.sh
+          python3 ./mach bootstrap-gstreamer
           brew install gnu-tar
       - name: Release build
         run: |

.github/workflows/windows.yml

@@ -52,13 +52,6 @@ jobs:
           fetch-depth: 2
       - name: Copy to C drive
         run: cp D:\a C:\ -Recurse
-      - name: Install GStreamer
-        shell: powershell
-        run: |
-          Start-BitsTransfer -Source https://gstreamer.freedesktop.org/data/pkg/windows/1.16.0/gstreamer-1.0-msvc-x86_64-1.16.0.msi
-          Start-Process msiexec.exe -Wait -ArgumentList "/i gstreamer-1.0-msvc-x86_64-1.16.0.msi /quiet /qn /norestart ADDLOCAL=ALL"
-          Start-BitsTransfer -Source https://gstreamer.freedesktop.org/data/pkg/windows/1.16.0/gstreamer-1.0-devel-msvc-x86_64-1.16.0.msi
-          Start-Process msiexec.exe -Wait -ArgumentList "/i gstreamer-1.0-devel-msvc-x86_64-1.16.0.msi /quiet /qn /norestart ADDLOCAL=ALL"
       - name: wix311-binaries
         shell: powershell
         run: |
@@ -72,6 +65,7 @@ jobs:
         run: |
           python -m pip install --upgrade pip virtualenv
           python mach fetch
+          python mach bootstrap-gstreamer
       - name: Release build
         working-directory: "C:\\a\\${{ github.event.repository.name }}\\${{ github.event.repository.name }}"
         run: python mach build --release --with-${{ env.LAYOUT }}