Refactor support for VDB customization scripts
Upon strartup, VDB now loads `~/.vdbinit.st`. If that file does not
exist, it loads default `vdbinit.st`. This default file also serves
as an example and documentation for available settings.
### 2019-02-14:
* Performance improvements to make VDB "snappier" especially when
debugging complex C++ codebases on slow(ish) systems such as
on *HiFive Unleashed* board.
### 2019-01-25:
* Major improvements in GDB console widget for setups where PTY
cannot be used (such as on Windows or when using GDB over SSH)
Most importantly, new console supports command completion
(when GDB supports MI command `-complete`)
### 2019-01-21:
* Support for arbitrary command to invoke GDB. This way one can use
VDB on a local workstation while the GDB itself runs on a remote
computer (server or development board, for example)
* Support for command completion on Windows.
### 2018-10-05:
* Add support multiple debugger layouts. Currently supported layouts
are "Single", "Eclipse" and "IDAPro".