add browse button for executable selection

1. Have a browse to button for the executable/image file
2. Convert Windows path delimiters to '/' or double escape them. Make it much easier to paste a path from Windows Explorer.
3. Have a mechanism for running (for example) openocd before firing up the rest of the stack? It would be nice to have all of this stored somewhere in a config file - maybe there is one.

I know I could script some of this but if it is all under gdbgui's control it can make sure processes shutdown correctly etc. on Ctrl+C

Thanks. Is there a secret sign to let you know I have contrbuted? :)

##### Describe your environment
* Operating system and version: WIndows 10 Pro
* gdbgui version (`gdbgui -v`): 0.11.00
* gdb version (`gdb -v`):
* browser: chrome
* python packages (`pip freeze`):
appdirs==1.4.3
backports.functools-lru-cache==1.4
beautifulsoup4==4.6.0
certifi==2018.1.18
chardet==3.0.4
click==6.7
colorama==0.3.9
cycler==0.10.0
Django==1.10.6
enum==0.4.6
enum-compat==0.0.2
enum34==1.1.6
eventlet==0.21.0
fasteners==0.14.1
Flask==0.12.2
Flask-Compress==1.4.0
Flask-SocketIO==2.9.3
funcsigs==1.0.2
future==0.16.0
fuzzywuzzy==0.16.0
gdbgui==0.11.0.0
gevent==1.2.2
greenlet==0.4.13
idna==2.6
image==1.5.5
intelhex==2.2.1
itsdangerous==0.24
Jinja==1.2
Jinja2==2.10
junit-xml==1.8
lockfile==0.12.2
MarkupSafe==1.0
matplotlib==2.1.0
mbed-cli==1.1.1
mbed-greentea==1.3.2
mbed-host-tests==1.3.0
mbed-ls==1.3.6
mercurial==4.4.2
mock==2.0.0
monotonic==1.4
myhdl==0.9.0
numpy==1.13.0
olefile==0.44
opencv-python==3.3.0.10
pbr==3.1.1
Pillow==4.0.0
prettytable==0.7.2
pygdbmi==0.8.2.0
Pygments==2.2.0
pyOCD==0.8.1a1
pyparsing==2.2.0
pypugjs==4.2.2
pyserial==3.4
python-dateutil==2.6.1
python-engineio==2.0.2
python-socketio==1.8.4
pytz==2017.3
pywinusb==0.4.2
PyYAML==3.12
requests==2.18.4
six==1.11.0
svgwrite==1.1.11
urllib3==1.22
websocket-client==0.46.0
Werkzeug==0.14.1

Start by tracing gdbgui's executable-selection UI and the process-startup path. Define separate completion checks for browsing and Windows path handling, optional pre-launch commands and configuration, and reliable cleanup on Ctrl+C; the issue names no files or tests to begin with.

Written by the indexing model from the issue text.