[feat] Add ReaderBack (#3821)

This implements a reasonable facsimile of going back on Android.

The back button first goes back in a history of visited pages.
When there's no history left, it closes the app.

Fixes #3816.

frontend/util.lua

@@ -95,6 +95,46 @@ function util.secondsToClock(seconds, withoutSeconds)
     end
 end
 
+--[[--
+Compares values in two different tables.
+
+Source: <a href="https://stackoverflow.com/a/32660766/2470572">https://stackoverflow.com/a/32660766/2470572</a>
+]]
+---- @param o1 Lua table
+---- @param o2 Lua table
+---- @bool ignore_mt
+---- @treturn boolean
+function util.tableEquals(o1, o2, ignore_mt)
+    if o1 == o2 then return true end
+    local o1Type = type(o1)
+    local o2Type = type(o2)
+    if o1Type ~= o2Type then return false end
+    if o1Type ~= 'table' then return false end
+
+    if not ignore_mt then
+        local mt1 = getmetatable(o1)
+        if mt1 and mt1.__eq then
+            --compare using built in method
+            return o1 == o2
+        end
+    end
+
+    local keySet = {}
+
+    for key1, value1 in pairs(o1) do
+        local value2 = o2[key1]
+        if value2 == nil or util.tableEquals(value1, value2, ignore_mt) == false then
+            return false
+        end
+        keySet[key1] = true
+    end
+
+    for key2, _ in pairs(o2) do
+        if not keySet[key2] then return false end
+    end
+    return true
+end
+
 --- Returns number of keys in a table.
 ---- @param T Lua table
 ---- @treturn int number of keys in table T