Fix edit theme dialog title logic (#2205)

* Fix edit theme dialog title logic

As you can see from the code and the actual behavior, both share the
same code whether adding or editing a theme. So why does the title show
up as "Edit Theme" no matter what? It does seem strange in a way.

This feature appears to have been added in v2.0.0, but no one seems to
have cared about the issue during the two years.

Nonetheless, when I realized this recently, I decided to fix it.
Hopefully this will make things clearer for those users who may not be
able to tell the difference.

Now, when the "Add theme" button is clicked, the dialog title will
show "Add theme". If you right-click on a user-created theme, the dialog
title will show "Edit theme".

* Apply suggestions from lyswhut

Simpler code

Co-authored-by: lyswhut <lyswhut@qq.com>

---------

Co-authored-by: lyswhut <lyswhut@qq.com>

src/renderer/views/Setting/components/ThemeEditModal/index.vue

@@ -1,7 +1,7 @@
 <template>
   <material-modal :show="modelValue" max-height="90%" teleport="#view" @close="handleCancel">
     <main :class="$style.main">
-      <h2>{{ $t('theme_edit_modal__title') }}</h2>
+      <h2>{{ themeId ? $t('theme_edit_modal__title_edit') : $t('theme_edit_modal__title_add') }}</h2>
       <div class="scroll" :class="$style.content">
         <div :class="[$style.group, $style.base]">
           <div :class="$style.groupContent">