Fix panel drag and drop and applet drawing outside panel

This fixes:

• Show desktop + other icons drawing the top highlight outside the panel borders
• Panel content overlapping toolbox
• Added margin to the left and right to avoid icons too close to borders
• Show desktop icon taking more horizontal space than needed
• Panel margin highlight drawn outside the panel
• Accidental drag and drop from empty spaces between applets collapsing them and changing the order randomly
• Hover highlight for applets being misaligned
• Pressing/releasing applets would move them some pixels to the left/right instantly
• Fixed JS errors in console when you drag/drop applets
• Couldn't click toolbox

BUG:475216 FIXED-IN: 6.0