Switch to Search component in tag selector and support X/Esc to clear

This moves debouncing into the search component and adds cancel behavior
from the XUL search textbox. For now, this uses the X button from
Firefox.

scss/components/_search.scss

@@ -0,0 +1,21 @@
+.search {
+	position: relative;
+}
+
+.search input {
+	-moz-appearance: searchfield;
+	flex: 1 0;
+	min-width: 40px;
+	height: 24px;
+	padding-left: 4px;
+}
+
+.search .search-cancel-button {
+	background-image: url(chrome://global/skin/icons/searchfield-cancel.svg);
+	position: absolute;
+	width: 14px;
+	height: 14px;
+	top: 6px;
+	right: 6px;
+	cursor: default;
+}

scss/components/_tag-selector.scss

@@ -24,6 +24,13 @@
 	height: 100%;
 }
 
+.tag-selector-list {
+	list-style: none;
+	display: inline-block;
+	margin: 0;
+	padding: 0;
+}
+
 .tag-selector-filter-container {
 	height: auto;
 	flex: 0 0 1em;
@@ -32,16 +39,9 @@
 	padding: 0.125em 0 0.125em 0.5em;
 }
 
-.tag-selector-list {
-	list-style: none;
-	display: inline-block;
-	margin: 0;
-	padding: 0;
-}
-
-.tag-selector-filter {
+.tag-selector-filter-container .search {
+	display: flex;
 	flex: 1 0;
-	min-width: 40px;
 }
 
 .tag-selector-actions {