R1:d42d69aef699

Clean up some PHUI/PHUIX button behaviors

Summary:
Ref T12733. Some minor issues:

- The `strlen(...)` test against `$this->text` fails if a caller does something like `setText(array(...))`. This is rare, but used in `DiffusionBrowseController`, from D15487.
- Add PHUIX examples for icon-only buttons.
- Remove unused `SIMPLE` constant now that no callsites remain.

Test Plan:
- Viewed a directory in Diffusion's "Browse" view in a Git repository, no longer saw a warning / error log.
- Viewed…