#417 Support for .idx file used for .tap-files

[slc]

Specs is as follows:

• Everything after a semicolon on a line is ignored
• Offset is an offset into the actual file
• Offset can be written in hex (either by preceding 0x notation or trailing h, leading zeroes allowed), or as decimal
• The offset number is ended by either space or tab
• Following the offset is the string that will be shown, a semicolon will terminate that string
• Each index entry is terminated either by \n or \r\n

An index should be easy to navigate through without triggering a new auto-load, as it is not only meant to make it easier with multi game-tapes, but also multiload games which requires you to forward or rewind the tape during game play.

[slc]

Clarification, a semicolon is of course not necessary to terminate the string. It's not needed unless you want a comment behind the string for whatever reason. It will terminate also on a newline symbol :) Attached a game that can be used for testing.

[gpz]

r40761 adds two functions required for this

autostart_set_initial_tap_offset(offset) can be called to set the offset for autostarting
tap_seek_to_offset(tape_image, offset) can be called to seek to a certain offset

perhaps we need a generic datasette_seek_to_offset that can be called without needing the tapeimage pointer