OPDSbrowser refactoring (#9703)

Refactoring:
-removed duplicated code
-removed passing of username/password through all the modules
-logical order of the methods
-some optimizing and drying
-comments

New features:
-subcatalog link can be saved to the list of servers (eg: direct link to the catalog of books by an author)
-more book information can be fetched and shown (Book information)
-fixed access to the Standard library (by @KGKopli, closes #9372)

plugins/opds.koplugin/main.lua

@@ -43,7 +43,6 @@ function OPDS:addToMainMenu(menu_items)
     if not self.ui.view then
         menu_items.opds = {
             text = _("OPDS catalog"),
-            sorting_hint = "search",
             callback = function() self:showCatalog() end
         }
     end

plugins/opds.koplugin/opdsparser.lua

@@ -72,6 +72,7 @@ function OPDSParser:createFlatXTable(xlex, curr_element)
 end
 
 function OPDSParser:parse(text)
+    text = text:gsub("<%?xml%-stylesheet.-%?>", "")
     -- luxl doesn't handle XML comments, so strip them
     text = text:gsub("<!%-%-.-%-%->", "")
     -- luxl is also particular about the syntax for self-closing, empty & orphaned tags...

spec/unit/opds_spec.lua

@@ -327,7 +327,7 @@ describe("OPDS module #nocov", function()
                 local item_table = OPDSBrowser:genItemTableFromCatalog(catalog, "https://www.gutenberg.org/ebooks.opds/?format=opds")
 
                 assert.truthy(item_table)
-                assert.are.same(item_table[1].text, "Search")
+                assert.are.same(item_table[1].text, "\u{f002} " .. "Search")
             end)
             it("should generate URL on rel=subsection", function()
                 local catalog = OPDSParser:parse(navigation_sample)