[terminal] add padding to the left

[svenefftinge]

The terminal should get some space on the left-hand side as soon as
xtermjs/xterm.js#1208 is merged and consumed by us.

[svenefftinge]

I see a comment in the code https://github.com/theia-ide/theia/blob/master/packages/terminal/src/browser/terminal.css#L21

But I don't really understand what the issue is. At least I could move the padding-leftup to the .terminal-container class it looks good.

[mofux]

@svenefftinge I think you are right, adding a left padding to the parent container should work fine as of now. xtermjs/xterm.js#1208 is especially useful if you want to maintain a padding to the right, top or bottom edges while still having the scrollbar of the viewport filling the whole space.