feature: provide a setup configurator for gitbash

[ui] add custom shell command button to general options panel which lets the user choose from available configurators (currently one: gitbash)
[ui] minor relayout of general options panel (f.e. merged 3 default buttons into a single "reset to defaults" button)
Merge pull request #47 from markiewb/feature/gitbashconfigurator

feature: provide a setup configurator for gitbash