Adjustments for PETSCII mode drawing

[nail7]

Color limitations

In PETSCII mode, iCE color shouldn't apply, since any of the 16 colors can be selected as background color. However, PETSCII only allows for one uniform background color for the entire screen. Therefore it should not be possible to select the background color on a block by block basis.

Keyboard mapping

ASCII and PETSCII have common glyphs, but their position in the character set are different. It is currently close to impossible to even write a sentence in PETSCII mode, since the keyboard is mapped to the positions in the ASCII character set.
There are several ways to introduce a keyboard mapping for this purpose. The emulator VICE offers positional and symbolic keymaps.
In this particular case it might be enough to simply map identical glyphs properly and include the remaining graphical characters by way of the 20 available F-Key character sets. These character sets should have their own default in PETSCII mode.

Font selection

PETSCII offers two different font sets, shifted (upper and lowercase letters) and unshifted (uppercase letters only, additional graphic characters). .seq loading already implements the control code that defines which font is to be used. There is currently no way to change that setting though. The PETSCII fonts should be added to the View > Font menu as 'Commodore 64', 'Shifted' and 'Unshifted'.
(The reason to add them as 'Commodore 64' and not as 'PETSCII' is that other PETSCII systems like the Commodore VIC-20 have a different font.)