Have more controls in the speed read

### ⚠️ This issue respects the following points: ⚠️

- [X] This is a **enhancement/feature request**. Not a [bug or question](https://github.com/gsantner/markor/issues/new/choose).
- [X] The topic is **not** already reported at [Issues](https://github.com/gsantner/markor/issues?q=#js-issues-search). _(I've searched it)_.
- [X] Markor **is** up to date. See [Releases](https://github.com/gsantner/markor/tags) for the latest version. Updates are available from [F-Droid](https://f-droid.org/en/packages/net.gsantner.markor/) and GitHub.
- [X] The wanted feature/enhancement is not present in the latest development version (git master). (Please [download](https://nightly.link/gsantner/markor/workflows/build-android-project/master) and try the test version of Markor, named **Marder**. Don't worry; Markor and Marder appear as completely separate applications. You can install both side-by-side, and Markor's settings are not touched. If your desired feature is present, you don't need to open this issue. The change will be part of the next Markor update.)

### Description

1. Split +100 and -100 to two buttons
2. Add a slider to control the speed
3. Add an option to increase the time on a word by its number of characters, with definition on the increased time per character

### Information

Android version: irrelevant
Device: irrelevant
App Version: 2.10.9

### Source

F-Droid

### Format / File type

Not specific

### Additional info / Log
When the words are fewer than 100, “-100” doesn't work.

Start by locating Markor's speed-read screen and the existing +100/-100 controls. Review how word timing is calculated, then define the separate controls, speed slider, and per-character timing option described in the issue; done means all requested controls work, including when fewer than 100 words remain.

Written by the indexing model from the issue text.