Refactor Visual mode (`librz/core/tui/visual.c`) to use API instead of commands

[XVilka]

Currently, the code of Visual modes heavily relies on running commands instead of just calling the proper API functions. To reduce the overhead, code cruft, and code breakage in case of the command changes it is important to remove all those command calls and call the proper API directly.

[i] ℤ rg "rz_core_cmd[0f_]*\(" librz/core/tui/visual.c                                                                                                                                                                                 12:21:36 
683:          rz_core_cmd(core, buf, 0);
715:                  ch = rz_core_cmd0(core, cmd);
723:                  ch = rz_core_cmd0(core, cmd);
731:                  ch = rz_core_cmd0(core, cmd);
737:                  ch = rz_core_cmd0(core, cmd);
748:                  ch = rz_core_cmd0(core, cmd);
754:                  ch = rz_core_cmd0(core, cmd);
760:                  ch = rz_core_cmd0(core, cmd);
768:                  ch = rz_core_cmd0(core, cmd);
776:                  ch = rz_core_cmd0(core, cmd);
784:                  ch = rz_core_cmd0(core, cmd);
790:                  ch = rz_core_cmd0(core, cmd);
796:                  ch = rz_core_cmd0(core, cmd);
1992:                 rz_core_cmd0(core, "eco $(eco~...)");
1998:                 rz_core_cmd0(core, "s $(afb~...)");
2002:                 rz_core_cmd0(core, "s $(ii~...)");
2005:                 rz_core_cmd0(core, "s $(isq~...)");
2008:                 rz_core_cmd0(core, "s $(dm~...)");
2017:                 rz_core_cmdf(core, "sh~...");
2249:                         rz_core_cmd(core, buf, true);
2502:                         rz_core_cmd(core, buf, 0);
2529:                 rz_core_cmd(core, buf, 1);
2800:                         rz_core_cmd0(core, key_s);
2808:                         rz_core_cmd0(core, key_s);
2914:                                 rz_core_cmd(core, buf, 0);
2938:                                 rz_core_cmd(core, buf, 0);
3656:                 rz_core_cmd0(core, vi);
3670:         rz_core_cmd0(core, vi);
3712:                 rz_core_cmd0(core, cmd_str);
3841:         int ret = rz_core_cmd0(core, cmd);

[officialcjunior]

rz_core_cmd0(core, "pg");

For the above command, if this is what's getting executed, I can get an API just for that, I suppose. Same calls are in agraph as well.

[ret2libc]

I think this can probably be moved to a later release. It's just refactoring and we seem to have already a lot of things in our hands.