Add selector to sort tags by counter or alphabetically

![screencast](https://i.imgur.com/XUkTyRe.gif)

browser/main/SideNav/SideNav.styl

@@ -8,6 +8,29 @@
   display: flex
   flex-direction column
 
+.control
+  user-select none
+  height $control-height
+  font-size 12px
+  line-height 25px
+  display flex
+  color $ui-inactive-text-color
+
+.control-sortTagsBy
+  flex 1
+  padding-left 15px
+
+.control-sortTagsBy-select
+  appearance: none;
+  margin-left 5px
+  color $ui-inactive-text-color
+  padding 0
+  border none
+  background-color transparent
+  outline none
+  cursor pointer
+  font-size 12px
+
 .top
   padding-bottom 15px
 
@@ -82,6 +105,10 @@ body[data-theme="white"]
     background-color #f9f9f9
     color $ui-text-color
 
+  .control
+    background-color $ui-white-backgroundColor
+    border-color $ui-white-borderColor
+
 body[data-theme="dark"]
   .root, .root--folded
     border-right 1px solid $ui-dark-borderColor
@@ -91,7 +118,15 @@ body[data-theme="dark"]
   .top
     border-color $ui-dark-borderColor
 
+  .control
+    background-color $ui-dark-backgroundColor
+    border-color $ui-dark-borderColor
+
 body[data-theme="solarized-dark"]
   .root, .root--folded
     background-color $ui-solarized-dark-backgroundColor
     border-right 1px solid $ui-solarized-dark-borderColor
+
+  .control
+    background-color $ui-solarized-dark-backgroundColor
+    border-color $ui-solarized-dark-borderColor