Prepare esp-idf component for release

- Remove the alpha suffix
- Use Slint from the release tag not the branch.
  It would be awkward if the 1.2.0 esp-idf component would end up
  using Slint 1.3.0, so let's tie them together for now.

api/cpp/esp-idf/slint/CMakeLists.txt

@@ -29,7 +29,7 @@ else(SLINT_ESP_LOCAL_EXAMPLE)
     FetchContent_Declare(
         Slint
         GIT_REPOSITORY https://github.com/slint-ui/slint
-        GIT_TAG master #FIXME: this needs to be hardcoded to a version number
+        GIT_TAG v1.2.0
         SOURCE_SUBDIR api/cpp
     )
     FetchContent_MakeAvailable(Slint)

api/cpp/esp-idf/slint/idf_component.yml

@@ -4,7 +4,7 @@
 description: "Slint — declarative GUI toolkit"
 url: "https://slint.dev"
 license: "GPL-3.0-only OR LicenseRef-Slint-Royalty-free-1.1 OR LicenseRef-Slint-commercial"
-version: "1.2.0-alpha5"
+version: "1.2.0"
 discussion: "https://github.com/slint-ui/slint/discussions"
 documentation: "https://slint.dev/docs"
 repository: "https://github.com/slint-ui/slint"