This implements the suggested protocol discussed in
https://github.com/kovidgoyal/kitty/issues/7657.
Icons are handled by loading a cache. Both in-band PNG data, and
symbolic names are allowed.
Applications use a graphical ID to reference the icon both when
loading the cache, and when showing a notification.
* 'g' is the graphical ID
* 'n' is optional, and assigns a symbolic name to the icon
* 'p=icon' - the payload is icon PNG data. It needs to be base64
encoded, but this is *not* implied. I.e. the application *must* use
e=1 explicitly.
To load an icon (in-band PNG data):
printf '\e]99;g=123:p=icon;<base64-encoded-png-data>\e\\'
or (symbolic name)
printf '\e]99;g=123:n=firefox:p=icon;\e\\'
Of course, we can combine the two, assigning *both* a symbolic
name, *and* PNG data:
printf '\e]99;g=123:n=firefox:p=icon;<base64-encoded-png>\e\\'
Then, to use the icon in a notification:
printf '\e]99;g=123;this is a notification\e\\'
Foot also allows a *symbolic* icon to be defined and used at the same
time:
printf '\e]99;g=123:n=firefox;this is a notification\e\\'
This obviously won't work with PNG data, since it uses the payload
portion of the escape sequence.
