Fix core library import path

[jefferyto]

The gedit plugin working directory isn't documented, and as such can change from version to version. This is the root cause of #9.

This changes the core library path to be computed based on the location of shared.py, instead of assuming the plugin working directory. This should finally allow the plugin to use the bundled core library instead of having to install the library separately.

[jefferyto]

With the bundled core library updated, now would be great time to merge this :laughing:

This change allows the plugin to use the bundled core library, removing the need to install the core library separately and making it easier for new users to install and use the plugin.

[xuhdev]

Thanks!