add UI support for watchpoints

* add UI support for breakpoints that have the type `watchpoint`

use the gdb mi [`-break-watch` command](https://sourceware.org/gdb/onlinedocs/gdb/Set-Watchpoints.html) to add a UI element where appropriate (i.e. on expression or memory) to set watchpoint on memory/expression

* when *read from* (`-break-watch -r `)
* when *written to* (`-break-watch `)
* when *read from or written to* (`-break-watch -a `)