Widgets should be horizontally centered in the sidebar. Else they look
out of place (in particular the clock and the applications widgets).
Due to little tricks with the sidebar starting out of the screen to
hide rounded corners, this implies playing with paddings. The patch
decreases the widgets padding from 4 to 2 pixels, removes additionnal
padding on the right, and adds an out-of-screen padding to the widget
box to make up for the negative horizontal position of the sidebar.
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