fix #11

* canvas dialog is a bit too complex to have "Cancel" button revert any changes we may have applied. (It's not out of the realm of possibility, but for now it's easier just to use a nonstandard "Close" button to show the discrepancy)
* font dialog updates on change of radio button, so we just need a "Close" button there
* prefs dialog also too complex to do a revert on "Cancel"
* iemguis and gatom properly revert on "Cancel"