First stab at register support

[matsl]

What

First stab at register support

Why

Sharing the first stab at implementing register support of buttons for ideas and comments.

[matsl]

@rswgnu Added support for link-to-ibut

[matsl]

@rswgnu Added test cases to the PR. The test hypb-register-test--register-val-insert fails due to what it seems a problem with link-to-ibut. The setup is in the test. I will try to debug it further but maybe you can have a look?

[rswgnu]

Note that there are explicit and implicit buttons where the button source buffer does not have an attached file, so you'll need to handle those appropriately, i.e. links to them work only within the current Emacs session, which may be okay for use with registers which typically are not stored across sessions either.

[matsl]

Does not link-to-ebut require a file name?

How should we support the case for origin buffer with no filename? Shall we jump to button source and execute the action key instead of using link-to-ebut?

[rswgnu]

Yes, jump to the button source buffer and move point to the button text (label) and activate the button there. If the source buffer is no longer live, then raise an error.

[matsl]

Since insert-register inserts an ebut that the user acts on would we not need a new action type then for that behavior or let link-to-ebut and link-to-ibut also handle the buffer only case?

[matsl]

Added a test case for inserting an ebut and commented out the similar test case for ibut. The later test case looks like it breaks due to problem with link-to-ibut and having a red build seems contra productive at this stage.