Ability to add custom action buttons on the toolbar

### ⚠️ 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 and checked most recent issues)_.
- [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

Hello,
I think that the toolbar is a bit limited, it would be helpful if there was a way to add or edit action buttons.
I don't think this is unique to me, I tend to add symbols at the start of lines and bullet points to mark them for certain stuff and I format my notes in a certain way, on pc I'm able to add keybinds for that stuff so it feels bothersome to type it every time on mobile.

Kind Regards.

### Information

Android version: 15
Device (phone model): Samsung
App Version: v2.16.1

### Source

F-Droid

### Format / File type

Not specific

### Additional info / Log

```shell
-
```

The issue names no files, tests, or entry points. Start by locating the Android toolbar implementation and clarifying how users should add or edit action buttons. Done means the requested toolbar customization is available for inserting the user's recurring symbols, bullets, and formatting actions.

Written by the indexing model from the issue text.