viewSelector: Replace tab system with pages

The old tab system is no longer a good fit for the current design, so
replace most of the abstractions with a simpler page system.

https://bugzilla.gnome.org/show_bug.cgi?id=682109

data/theme/gnome-shell.css

@@ -648,16 +648,8 @@ StButton.popup-menu-item:insensitive {
     spacing: 1em;
 }
 
-#viewSelectorTabBar {
-    padding: 1em;
-}
-
 /* Search Box */
 
-#searchArea {
-    padding: 0px 24px;
-}
-
 #searchEntry {
     border-radius: 17px;
     width: 250px;
@@ -668,26 +660,6 @@ StButton.popup-menu-item:insensitive {
     color: #8d8f8a;
 }
 
-/* View Tabs */
-
-.view-tab-title {
-    color: #888a85;
-    font-size: 12pt;
-    font-weight: bold;
-    padding: 0px 0.75em;
-    height: 1.5em;
-}
-
-.view-tab-title:hover {
-    color: #bbb;
-}
-
-.view-tab-title:selected {
-    color: #000000;
-    background-color: #c2c7cd;
-    border-radius: 0.25em;
-}
-
 /* Search Results */
 
 #searchResults {