Improve the Preferences -> Plugins tab

[csnyulas]

There were several questions on the mailing list about how to find the relevant information about the installed plugins in Protege. In particular, people (naturally) thought that the Preferences -> Plugins tab should provide some information about the installed plugins.
I would suggest the following changes to the Preferences -> Plugins tab to improve user experience:

1. Provide a hint on where the information about installed plugins can be found (Help -> About). (Optional) We could also provide a hint about how to install plugins and/or update existing ones (File -> Check for plugins...). If we could make those references clickable, with the effect to execute those menu item actions, it would an extra bonus.
2. Make the text box for editing the location of the Plugin registry longer, so that (almost?) the entire path to the repository can be visible. The right end of the textbox should be in line with the end of the separator line above it, when the Preferences window is shown with its default dimensions. Ideally the textbox would increase/decrease when the preferences window is resized, but this is just a nice to have.
   

[csnyulas]

Regarding point 2. could you set the preferred size, and maybe min size of the pluginRegistryEditor, instead specifying the column number to be 50?

2036d94#diff-c4605a8b1e381dd9af8475c2f5d59557R56

Thanks!