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)

This simplifies the setup procedure for GitBash. You do not have to remember the commandline arguments for gitbash anymore.

![2014-03-22_16h49_34](https://f.cloud.github.com/assets/1857095/2491085/ae9c7316-b1da-11e3-9957-b5760cdbcf99.png)