added buttons for insert and replace module in the module editor
with a new bitmap. restored JP's original functionality for the insert
button, replace linked to the new code.
* backspace key now deletes segments and vias both while editing a new track and while
modifying old ones
* internal layers can be accessed via F5-F8
* when creating a new trace, switching between layers with the Pageup, PageDown, and
F5-F8 hotkeys inserts a via. (warning: i did not test this with blind vias; however, it
calls Other_Layer_Route, so should still work)
* spelling fix
dickelbeck