[zathura] Provide uni-characters accelerators for links

[Denis Bitouzé]

Hi,

currently, in order to open links that are shown in the document, one
has to:

1. press the "f" key on the keyboard,
2. type one the possibly multi-characters (numbers possibly > 9)
accelerators zathura highlights for the different links on the
current screen,
3. press `Enter'.

For instance, if the link I want to open is highlighted with `13',
I have to type:

1. f
2. 1
3. 3
4. `Enter'

It would be more efficient that:

1. pressing "f" key on the keyboard puts zathura in a special mode in
which all its shortcuts (but e.g. `escape' that let us exit this
mode) are disabled,
2. zathura highlights the different links on the current screen with
only uni-characters accelerators such as [0-9a-zA-Z],
3. just pressing one of these accelerator follows the corresponding link
and exit this special mode.

In such a case, if the link I want to open is highlighted with `d',
I have to type only:

1. f
2. d

Best regards.
--
Denis