Use branches

[kisvegabor]

Is your feature request related to a problem? Please describe.
Now we use a fixed, not released/tagged LVGL version. It's would better to allow selected LVGL's release/v*.* branches. (E.g. release/v9.2). It will make the project creator more future proof (no new release is need to update LVGL) and backward compatible too (allow creating projects for older LVGL versions too).

Describe the solution you'd like
Now we have a "branches" tag in the manifests that can be used to select one of the supported branches and use the related LVGL version.

Describe alternatives you've considered

None.

Additional context

See #20