feat(ui): add cons_show_warning() (issue #87)

The console had cons_show() for information and cons_show_error() for
failures, but nothing in between, so callers reporting a condition the
user should notice had to pick between hiding it and dressing it up as
an error.

cons_show_warning() prints through the new THEME_WARNING item, yellow
by default, and prefixes the text with "Warning: ". Themes that do not
set the "warning" colour fall back to that default, so the bundled
themes need no change. Logging stays with the caller, as it does for
the other console functions.

tests/unittests/ui/stub_ui.c

@@ -49,6 +49,18 @@ expect_any_cons_show_error(void)
     expect_any(cons_show_error, output);
 }
 
+void
+expect_cons_show_warning(char* expected)
+{
+    expect_string(cons_show_warning, output, expected);
+}
+
+void
+expect_any_cons_show_warning(void)
+{
+    expect_any(cons_show_warning, output);
+}
+
 void
 expect_win_println(char* message)
 {
@@ -845,6 +857,16 @@ cons_show_error(const char* const cmd, ...)
     va_end(args);
 }
 
+void
+cons_show_warning(const char* const msg, ...)
+{
+    va_list args;
+    va_start(args, msg);
+    vsnprintf(output, sizeof(output), msg, args);
+    check_expected(output);
+    va_end(args);
+}
+
 void
 cons_show_contacts(GSList* list)
 {

tests/unittests/ui/stub_ui.h

@@ -10,4 +10,6 @@ void expect_cons_show(char* expected);
 void expect_any_cons_show(void);
 void expect_cons_show_error(char* expected);
 void expect_any_cons_show_error(void);
+void expect_cons_show_warning(char* expected);
+void expect_any_cons_show_warning(void);
 void expect_win_println(char* message);