Minor clang-tidying

[biodranik]

No description provided.

[rtsisyk]

"Use constant base font height" and "minor clang-tidying" are two unrelated changes. The latter probably don't raise any questions and can be merged via a separate PR.

[biodranik]

@rtsisyk @vng PTAL, only minor tidying is left.

[vng]

Well, I find these changes strange. Yes, in terms of C++ some methods may be const, but not in terms of design logic.

How making const functions like Invalidate, SetXXX, AddXXX should help to understand the code better - IDK. I made a quick look and don't agree here.

[vng]

This short example shows how we should filter all tidy stuff.

class Foo
{
    class Impl
    {
        int m_state;
    public:
        void ChangeState() { ++m_state; }
    };

    Impl * m_impl;

public:
    // Aha, const, oops.
    void ChangeState() const { m_impl->ChangeState(); }
};