Annotations for watch and compute methods similar to @on for event handlers

[liancheng]

One pain point of mine about Textual magic methods like on_*, watch_*, and compute_* is that they are not refactoring-friendly. For example, while renaming a reactive variable using IDE refactoring features, the watch_* and compute_* methods cannot be covered, and I have to rename them manually.

The @on annotation is perfectly for solving this problem for event handlers: while designing a new event for my widget, renaming the event class no longer requires searching for all the relevant on_* methods and renaming them manually.

It would be great if we can have @watch and @compute annotations to replace watch_* and compute_* magic methods.