theme: Apply icon color through IconButtonTheme

The most conspicuous change this makes is to the "back" icon
in an AppBar, aka AppBar.leading.  Those have been a white or
black color from Material defaults; now they get our "icon" color.

This matches what's in Figma; and also matches the action icons,
for any app bars that have both.  Currently our only app bar with
an action is also the only one that lacks a "leading": it's on
ChooseAccountPage, which is at the root of navigation so there's
nowhere for a "back" icon to lead to.  But when we add actions in
other pages, the mismatch would become conspicuous.

This change also turns a handful of less-visible icon buttons
from white/black to the icon color: the lightbox's "copy link"
or play/pause buttons, and the show/hide-password button on the
username/password login page.  I think those changes are desirable
too.  All our other icon buttons that I can find already (either
in the source, or by browsing around the app) have a color specified,
and aren't affected by this change.

Author: gnprice

lib/widgets/theme.dart

@@ -36,6 +36,9 @@ ThemeData zulipThemeData(BuildContext context) {
     brightness: brightness,
     typography: zulipTypography(context),
     extensions: themeExtensions,
+    iconButtonTheme: IconButtonThemeData(style: IconButton.styleFrom(
+      foregroundColor: designVariables.icon,
+    )),
     appBarTheme: AppBarTheme(
       // Set these two fields to prevent a color change in [AppBar]s when
       // there is something scrolled under it. If an app bar hasn't been
@@ -45,10 +48,7 @@ ThemeData zulipThemeData(BuildContext context) {
       scrolledUnderElevation: 0,
       backgroundColor: designVariables.bgTopBar,
 
-      // TODO match layout to Figma
-      actionsIconTheme: IconThemeData(
-        color: designVariables.icon,
-      ),
+      // TODO match actions layout to Figma
 
       titleTextStyle: TextStyle(
         inherit: false,