Minor adjustments to event icons required after recent font changes.

We have a type of icon that is actually just text drawn inside of a border to represent a keyboard key, used on the footer to indicate available keymaps. With the old metafont I had made manual adjustments to some glyphs in order to make sure these were there, were consistently sized and positioned.

However we are now using multiple standard fonts and we don't want to make any manual changes so that they are easer to maintain, augment, and replace. This patch makes some adjustments to the event icons code to make them easier to read. The following shows how these icons look now in master at the top, while the bottom shows the result of this patch:

This is also a net reduction and simplification of the code.