Add coloring of statusbar.current tab in view

This theme color applies to the tab title text of the statusbar tab that
is currently shown.

The (somewhat confusingly named) `statusbar.active` theme color now
applies to all other tabs (before, it applied to all tabs).

Coloring of a tab that is highlighted/has new messages is done as before
using the `statusbar.new` theme color.

The default color is set to `cyan`, and thus causes no visible change
for users -- until modified.

src/config/theme.h

@@ -68,6 +68,7 @@ typedef enum {
     THEME_STATUS_TEXT,
     THEME_STATUS_BRACKET,
     THEME_STATUS_ACTIVE,
+    THEME_STATUS_CURRENT,
     THEME_STATUS_NEW,
     THEME_STATUS_TIME,
     THEME_ME,