I've always thought that SharePoint Designer is one of the most embarrassing crappy piece of software ever written, which caused me countless hours of frustration, but this is a bit too much. They should have dropped it altogether, not just mutilating it by cutting out the designer view. Anyway, enough ranting.
Here's a "quick and dirty" workaround to get the beloved List View Tools tab back.
Open the XsltListViewWebPart Tag properties (position the mouse over the <WebPartPages:XsltListViewWebPart> tag, right click, then click on Tag Properties or alternatively, on the bottom right of the screen, click on the breadcrumb <WebPartPages:XsltListViewWebPart> part, then click on Tag properties). A popup window will appear. Don't change anything (unless you actually need to) and click on the OK button. Et voilà, The List View Tools tab will immediately show up.
You will need to repeat the steps above every time you re-open the page and need to edit the webpart again.