feat: add placeholder for category search

src/views/admin/partials/categories/select-category.tpl

@@ -5,7 +5,7 @@
 				<span component="category-selector-selected">[[topic:thread_tools.select_category]]</span> <span class="caret"></span>
 			</button>
 			<div component="category-selector-search" class="hidden position-absolute">
-				<input type="text" class="form-control" autocomplete="off">
+				<input type="text" class="form-control" placeholder="[[search:type-to-search]]" autocomplete="off">
 			</div>
 			<ul component="category/list" class="dropdown-menu category-dropdown-menu" role="menu">
 				<li component="category/no-matches" role="presentation" class="category hidden">

src/views/admin/partials/category/selector-dropdown-content.tpl

@@ -10,7 +10,7 @@
     {{{ end }}}</span> <span class="caret"></span>
 </button>
 <div component="category-selector-search" class="hidden position-absolute">
-    <input type="text" class="form-control" autocomplete="off">
+    <input type="text" class="form-control" placeholder="[[search:type-to-search]]" autocomplete="off">
 </div>
 <ul component="category/list" class="dropdown-menu category-dropdown-menu" role="menu">
     <li component="category/no-matches" role="presentation" class="category hidden">

src/views/admin/partials/groups/privileges-select-category.tpl

@@ -3,7 +3,7 @@
 		<span>[[admin/manage/groups:privileges]]</span> <span class="caret"></span>
 	</button>
 	<div component="category-selector-search" class="hidden position-absolute">
-		<input type="text" class="form-control" autocomplete="off">
+		<input type="text" class="form-control" placeholder="[[search:type-to-search]]" autocomplete="off">
 	</div>
 	<ul component="category/list" class="dropdown-menu category-dropdown-menu dropdown-menu-end" role="menu">
 		<li component="category/no-matches" role="presentation" class="category hidden">