support frame filters

[GitMensch]

I thought to have read somewhere that frame filters are enabled, but actually they aren't. Enabling one and then executing "bt" shows the expected result, but after step (where we read the stack) the frames are still shown as before. Enabling those manually on the debugger commands -enable-frame-filters (either on the debugging console or via autocmds) does activate it, but shows strange results (file and some other parts undefined).

I plan to tackle possible issues (frame filters can drop functions, filenames, addresses, and also "hide" stack entries) and - as soon as that works with filters I use - consider to add a boolean setting for to enable frame filters when using gdb (defaulting to true).

Any insights about other debuggers than GDB would be nice, I don't know if this works for non-gdb the same way.