No way to manually set path to Zim executable

[Michael Mulqueen]

There is currently no way to manually set the path to the Zim executable.

This is problematic in certain circumstances:
* Testing -- I test against the Zim trunk as well as the latest release, but the Zim trunk lives in my home directory (which is not on the PATH list)
* Odd configurations -- if for some reason Zim Capture can't find the executable, there is currently no way to override it as a fix
* Windows -- the executable locator in Zim Capture for Windows is primitive, so if it's not in C:\Program Files\ then Zim won't be found, manually setting the path could be a fix until I get round to a proper locator for Windows
* Mac OS X -- there is currently no OS X support, but this could be worked around if the path to the executable could be set manually

Fix: I'll probably set up a variable in about:config and if that isn't null then that will override any auto-detection. Eventually, I will be adding a preferences dialogue box.