fix(color): modifying model labels may corrupt model yaml files

[philmoz]

The change to make the model screens and widget data dynamic missed the fact that changing model labels causes reading and writing of model YAML files other than the currently loaded model.

Introduced in #6782

This is handled by dynamically allocating a ModelData buffer to read into and write from.
However the model screen and widget data no longer lives in the ModelData structure; but in global variables.

When reading other model files to update the labels, this would overwrite the screen and widget data for the currently active model. This data could then be written back to the active model file overwriting the correct settings.

It is currently not possible to store the screen and widget data in the ModelData structure as the system that generates the firmware YAML parser can only handle simple data types.

To avoid creating and managing even more temporary buffers this PR changes the logic for updating labels.

Changes:

• when reading labels from a model file, only the header section (PartialModel) is loaded into a temporary buffer.
• when a model (other than the active model) needs label changes the code reads the header from the model file into a PartialModel structure, updates the labels and then creates a temporary yaml file with this new header. It then reads the original model yaml file, skips the old header and block copies the rest of the original file to the temporary file. Once done the original is deleted and the temporary file is renamed.

This removes the need for dynamically allocated ModelData buffers (the PartialModel buffers are very small and live on the stack).
It reduces the overhead by not reading the entire model yaml just to get the labels from the header section.
It no longer touches any of the active model data structures.