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

src/applications/uiexample/examples/PHUIXComponentsExample.php

@@ -72,6 +72,23 @@ final class PHUIXComponentsExample extends PhabricatorUIExample {
         'color' => PHUIButtonView::GREEN,
         'text' => pht('Environmental!'),
       ),
+      array(
+        'icon' => 'fa-cog',
+      ),
+      array(
+        'icon' => 'fa-cog',
+        'type' => PHUIButtonView::BUTTONTYPE_SIMPLE,
+      ),
+      array(
+        'text' => array('2 + 2', ' ', '=', ' ', '4'),
+      ),
+      array(
+        'color' => PHUIButtonView::GREY,
+        'text' => pht('Cancel'),
+      ),
+      array(
+        'text' => array('<strong />'),
+      ),
     );
 
     foreach ($buttons as $spec) {

src/view/phui/PHUIButtonView.php

@@ -5,7 +5,6 @@ final class PHUIButtonView extends AphrontTagView {
   const GREEN = 'green';
   const GREY = 'grey';
   const DISABLED = 'disabled';
-  const SIMPLE = 'simple';
 
   const SMALL = 'small';
   const BIG = 'big';
@@ -175,7 +174,7 @@ final class PHUIButtonView extends AphrontTagView {
       $classes[] = 'has-icon';
     }
 
-    if (strlen($this->text)) {
+    if ($this->text !== null) {
       $classes[] = 'has-text';
     }
 
@@ -238,7 +237,7 @@ final class PHUIButtonView extends AphrontTagView {
         $this->subtext);
       }
 
-      if (strlen($this->text)) {
+      if ($this->text !== null) {
         $text = phutil_tag(
           'div',
           array(