[UX] Add Device:openLink() stub for Linux SDL (#4822)

References #4821.

frontend/apps/reader/modules/readerlink.lua

@@ -614,6 +614,11 @@ function ReaderLink:onGotoLink(link, neglect_current_location, allow_footnote_po
         return true
     end
 
+    -- try opening link in native browser
+    if Device:openLink(link_url) then
+        return true
+    end
+
     -- Not supported
     UIManager:show(InfoMessage:new{
         text = T(_("Invalid or external link:\n%1"), link_url),

frontend/device/generic/device.lua

@@ -64,6 +64,8 @@ local Device = {
 
     -- set to yes on devices that support over-the-air incremental updates.
     hasOTAUpdates = no,
+
+    openLink = no,
 }
 
 function Device:new(o)

frontend/device/sdl/device.lua

@@ -17,6 +17,10 @@ local Device = Generic:new{
     needsScreenRefreshAfterResume = no,
     hasColorScreen = yes,
     hasEinkScreen = no,
+    openLink = function(self, link)
+        if not link or type(link) ~= "string" then return end
+        return os.execute("xdg-open '"..link.."'") == 0
+    end,
 }
 
 local AppImage = Device:new{

frontend/ui/otamanager.lua

@@ -236,7 +236,7 @@ function OTAManager:fetchAndProcessUpdate()
                             android.notification(T(_("Downloading %1"), ota_package))
                         end
                     elseif Device:isSDL() then
-                        os.execute("xdg-open '"..link.."'")
+                        Device:openLink(link)
                     end
                 end
             })