Makefile improvements

[iam28th]

Greetings! There are some minor additions I'd like to make to the building process; I know that CoLoRd is currently not under active development, but hopefully it will still be of some help (I briefly checked, and it looks like similar changes -- if you like them:) -- can be implemented in other repositories).

PR consists of 2 commits:

1. The first one cleans up some missed files.

(a spoiler with the picture to see what files these are; .gitignore was temporarily deleted to make them visible)

2. The second is more of a convenience to simplify development workflow. It allows make to automatically detect when a .cpp files includes a recently modified header, and rebuild the corresponding object. Previously changes in .h files were not picked up, which could lead to unexpected stuff when objects contained outdated logic; and the only solution to such problem would be to make clean and recompile everything anew every time. Now such situations will not happen when editing files in src/colord.