Use more standard format for displaying keyboard shortcuts

This affects both the tooltips on tools and the help dialog.
"CTRL + SHIFT + N" is ugly, let's instead use "Ctrl + Shift + N".
Note that this was overriden for Macs, that behavior is not changed.

Change-Id: Ifd1e8e01e5bee5bf7bc82731d5bea27794c6dfb9