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 PHUIX Components UI examples.
• Grepped for ::SIMPLE.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12733

Differential Revision: https://secure.phabricator.com/D18065