Piano Roll Label / Tooltip Improvements

**Edited August 25th @tresf, fixed title and description**
- [ ] Make Piano Roll Label easier to read
- [x] Tooltip mentions "volume of a step" which doesn't apply to piano roll (fixed per #2655)
## 

**Original bug report**

in context of several tickets (#1839 , #1935 among others) The issue of general readability need focus.

> Removed my own opinions on readability and design.. tl;dr

Here is a couple of imgs where lmms is in need of general design-changes


Add to this thread everything that you see as an issue of
- visibility (Like: text obscuring or being obscured
- readability (Like:
- insufficient color differences between foreground and background.
- Font-size/type etc

And anything else that has relations to enhancing any visual information to the user.

adding #2209

Begin with the attached Piano Roll screenshots and the unchecked label-readability item; the issue names no source file or test. Done means the Piano Roll label is easier to read, while the tooltip wording item is already marked fixed.

Written by the indexing model from the issue text.