API: improved `GDBFrame >> #file` ...to try to resolve relative paths using source directories. GDB should do that but somethimes it gets confused so try ourselves in that case.

API: added methods for querying source directories This is an API equivalent of `show directories` command.

GNI/MI: added custom inspector view for viewing GDB/MI comminicatication trace ...to ease debugging/exploring GDB.MI. `GDBSessionRecord*` classes have been renamed to `GDBMITrace*` to avoid (future) naming confusion with event recording feature in GDB itself (reverse debugging).

Fixed `GDBVariableObject >> valueFormatted:` to work even if python pretty printing is enable If pretty printing is enabled, GDB seem to ignore `-f` parameter given to `-var-evaluate-expression`. To work around this, temporarily set variable visualizer to `None` (effectively disabling pretty printing), evaluate and then set it back to default.

API: added methods for querying debugger and target features * `GDBDebugger >> features:` to query a list of supported features * `GDBDebugger >> hasFeature:` to check if the debugger supports given feature. * `GDBDebugger >> ensureFeature:` to guard against missing feature These methods handle both, debugger and target features. Symbolic constants for features (known at the time) are defined in `GDBFeatures` pool. See [1] for documentation on features. [1]: https://sourceware.org/gdb/onlinedocs/gdb/GDB_002fMI-Support-Commands.html

Variable objecys: added `GDBVariableObject >> #visualizer` and `#visualizer:`

Variable objects: added `GDBVariableObject >> #duplicate` that creates a copy of that object.

Variable objects: keep track of frame in which variable object has been created

Variable objects: added `#path` returning an absolute expression to access the value in source langiage ...as opositte to `#expression` with return a parent-relative expression.

Variable objects: invalidate variable object when thread on inferior terminates ...and delete the variable and all its children (by means of `-var-delete`).