Handling missing schemas and keys

Considering the fact that unity-tweak-tool is hard-coding the schemas and keys it is handling, we need a function that informs beforehand whether it is installed or otherwise. This is critical because missing schemas and keys seg-faults UTT and that is not a pleasant experience for the users. What we need is a system that informs the user that a particular package from where we are sourcing the schemas and keys is not installed.

I propose the following mechanism to achieve the above:

1. Check if a schema is missing.
2. Check if a particular key is missing in a schema.
3. If a schema is missing, the UI components dealing with that schema should be unsensitized and the tooltips should be updated informing the user that a package needs to be installed if that UI component needs to be interacted with. We can achieve the above with the help of a function in, say, ui.py.
4. If a key is missing, proceed similarly to number 3.