Add filter to GDB's register buffer

Add filter to GDB's register buffer

* lisp/progmodes/gdb-mi.el (gdb-registers-enable-filter)
(gdb-registers-filter-pattern-list): New custom options.
(gdb-header-click-event-handler, gdb-registers-toggle-filter): New
functions.
(gdb-header-click-event-handler): Only add a register if it passes the
filter.
(gdb-registers-mode-map): New keybinding for toggling the filter.
(gdb-registers-header): New buttons on the header line for the
filter (bug#39179).