Termdebug: add Frame command to switch the frame

[GitMensch]

That's actually a quite easy addition, so I'll likely work on this sometime in the next weeks. The goal is to reduce the amount of things one as to move to the gdb Window or use :call TermDebugSendCommand() for.

Specification:

• when arg is a number: execute the frame N command
• when arg is "u[p] [N]"/"d[own] [N]" execute the up [N] / down [N] commands
• when arg is "s[elected]" query the current selected frame S and execute the matching frame S command
• when arg is "f[ile]" try to find the most inner frame F matching the file opened in the current buffer and execute the matching frame F command
• in every other case: execute the frame function arg command

If this is unlikely to be merged then please say up-front, otherwise expect a pull request in one or two weeks :-)
If this is something you' like to see, please just react to this issue.