TYPO3 Core

Problem:
The current workspace selector is a select HTML element. When changing a workspace it reloads the whole backend.
The checkbox for en/disabling the frontend preview of workspaces is only available in draft workspaces. Toggling it's value also reloads the whole backend.
When the backend gets reloaded you end up in the module that is set as your start up module.
The whole workspace selector, due to its select element also takes up a lot of room.

Solution:
Turn the workspace selector into a menu like clear cache, shortcuts and so on.

Do not reload the whole backend when changing the WS. Instead, just reload the module menu and the content iframe. When the user switches to a workspace and he's in a submodule that is not a submodule of the web module, redirect him to the web->page submodule as otherwise he could be left in a module that is not available in the new WS (like the file module or the EM in Draft WS).

When the user is in a web submodule already when changing the WS, the content iframe will be reloaded and the user stays at the same page he was before instead of going to his startup module.

The frontend preview selector is now always available and doesn't cause a reload of the whole backend either when toggling the setting.

As the WS icon is used for the menu now, there's also an entry that allows to go to the WS module which was the purpose of the icon before.

Notes:
This menu was also present in alt_main_new.php
Copy the checkmark image into typo3/gfx/.
The old js file "workspaces.js" will be removed.
Localization will be added later (after figuring out into which lang file the labels should go).
(issue imported from #M9813)