Fix Pipeline - refactor code and update pipeline file

As of commit 28441164 , pipeline doesn't run completely. Part of the reason being incorrect command for clang_tidy, and other being the fact that the code actually does fail CI test.

In this pull request, I've made changes to CI file and refactored code so as to result in successful pipeline

• Rust codebase changes are based on clippy warnings and suggestions, and hence only bare minimum changes are included. Changes include removing unused mutable declarations, removing unused variables, collapsing nested matches or conditionals (wherever applicable).
• Rust code formatted with cargo fmt
• clang_tidy step in pipeline now runs cmake to generate compile commands database, and will check only cpp files

A sample run of this pipeline can be seen at my personal GitLab account. Link : https://gitlab.com/flyingcakes/ikona-test/-/pipelines/467056612

Changes that need a verification from maintainer

• I have dropped variable to suppress warning. This is probably intended by the original developer, but I'm listing separately to confirm with maintainer : src/rs/src/capi/capi.rs line 634

• I added a return 0 to a non void function that could possibly exit without returning anything. I'm not sure if the function is actually intended to exit without return value, but doing so gives a clang_tidy error. This needs a confirmation as to whether return 0 won't break application logic : src/thememodel.cpp line 82