Switch tabs using Ctrl+PgUp/Down, reorder tabs using Ctrl+Sh+PgUp/Down

[exyi]

The keymapping matches web browsers, VS Code and other software with tabs.

This patch also requires that scrolling shortcuts do not accept modifier keys, so ScrollArea.KeyDown is strictened accordingly.

[shpaass]

Hello @exyi!
Could you please separate the refactoring and new functionality into different commits? It would help with potential debugging in the future.
If it's too much work, please let us know.

[shpaass]

I'm not sure how a user would be able to discover these shortcuts.
As one of the options, we could have a file Docs/Shortcuts.md that lists all the shortcuts.
In this PR, you could either list all the shortcuts or put just the ones you added -- it's fine to not add them all at once.

[exyi]

Hi! Sorry for the delay, I have just separated changes in ScrollArea and then the tab keymapping. I have added the Docs/Shortcuts.md file and also listed the other shortcuts which I could find. If you know of some other, let me know, I can add them too, or we can add them later

[veger]

It would be nice to add a link to the new Shortcuts.md in the main page/README, so users will (hopefully) find it

[shpaass]

It would be nice to add a link to the new Shortcuts.md in the main page/README, so users will (hopefully) find it

I'll handle that after we merge this PR.

[shpaass]

I'll handle that after we merge this PR.

Added the link to Shortcuts in Readme.