Remove the border around the tabs in Lokalize (QMdiSubWindows)

The tab contents previously had a 1px border around it, which is not needed because there is already a divider along the bottom above the status bar. The extra borders were also separating the tab itself from the tab's content, and were visible when the window was not maximised, meaning the window decoration borders were shown as well as the qmdisubwindows at the edge of the Lokalize window.

The bottom left corner sometimes has a border with a curved corner along it, sometimes not, I'm not sure why the QMdiSubWindow border only sometimes showed there. This fixes it.

After (maximised):

After (windowed):

Before: