Code style: add missed explicit 'this->'

I've made a clang-tidy based tool that can enforce both explicit and implicit this-style. I understand that this is a pretty huge MR and it will be hard to check it manually, so I wrote the article about how it works and why explicit this conversion is really safe. Link to tool's source code is at the bottom, with some effort you can build and use it as a part of your pre-merge check or just make some local tests if you're afraid of this big MR.
Because implicit this looks more sane for me I've made this conversion too, the branch with it is here. Keep in mind that, as with any other clang-tidy check, ifdef-removed parts are not checked, so for a full coverage it should be run on different configurations. This particular MR is based on Ubuntu Debug configuration.
Oh, it's already behind the master. If it will be approved I will do rebase.