gui2/tscreenshot_notification: New GUI2 dialog shown when taking screenshots

It includes the following features:

 * A cleaner and more structured message display
 * Buttons to copy the screenshot file path to clipboard or open it with
   an external application defined by the platform
 * An additional button to browse to the screenshots folder using an
   external file manager defined by the platform

It also breaks the scons build. This will be addressed by the next
commit.
scons: Link libwesnoth_extras.a into wesnoth twice

Solves linker errors caused by hotkey/command_executor.o's newly-gained
dependency upon gui/dialogs/screenshot_notification.o.
Update changelogs and RELEASE_NOTES
Merge branch 'staging/feature/screenshot-notifications'