Added hex viewer

[chrisgleissner]

Features

• Added hex viewer (https://github.com/GideonZ/1541ultimate/issues/320), using a format popular with various C64 Machine Code Monitors: 4 byte offset, 8 bytes data, first in hex, then in the Ultimate's UI character set with dots for bytes that can't be rendered.
• Support new editor shortcuts: Home/F2: start of file. End/F8: End of file. This facilitates navigating across huge files.

Design Notes

• The decision about which view to use for opening a file was deliberately left to the user. There may be legit use cases to open a text file in hex view, and vice versa.
• The firmware resulting from this PR was successfully flashed to a 64 Ultimate Elite, revision 1.4. All testing was done on this machine.
• The offset rolls over from FFFF to 0000 if browsing a file larger than 64KiB. This should be OK as it mostly concerns browsing D64 and similar large files.

Examples

New "Hex View" entry in context-sensitive menu:

New "Hex View":

Same file opened via existing "View" which was left unchanged. In this example, byte 0x00 at offset 0x3b terminates the string rendering:

New "Hex View" using sample file containg contiguous sequence of numbers from 1 through 255:

[chrisgleissner]

The branch feature/320-hex of this PR has been merged into the new branch chrisgleissner-feature/320-hex and the merge conflicts have been resolved there. Pending on that new branch are adjustments to the build files for the non-U64 builds which @GideonZ will take care of as discussed - many thanks.