feat: open editable files on the left, non editable on the right

[cguedes]

Fixes #131 and fixes #128

With this PR PDF files are opened in the right panel, while the others open on the left. Note that the split file feature is still available as an action from the open files tree.

This PR also adds unit tests for the atom fileActions (and indirectly to the related core atoms). I've added also an action atom to close all files closeAllFilesAtom.

[hammer]

Thanks I think this reduction in feature complexity is a good one!

A thought while trying out the PR: I wonder if there's an easy way to add a way to close all reading tabs, similar to clicking on the icons on the left to collapse the left sidebar? I guess we're starting to get too many kinds of UI elements with a left sidebar, a second reading pane, and a right sidebar, and it would be good to unify or standardize them. No action to take now, just thinking out loud.

[cguedes]

A thought while trying out the PR: I wonder if there's an easy way to add a way to close all reading tabs, similar to clicking on the icons on the left to collapse the left sidebar? I guess we're starting to get too many kinds of UI elements with a left sidebar, a second reading pane, and a right sidebar, and it would be good to unify or standardize them. No action to take now, just thinking out loud.

We can have an action in the "OPEN FILES" panel header like vs code.

This is very easy to implement as we already have the operation closeAllFilesAtom. Should I create a task for this?

[hammer]

Should I create a task for this?

Sure!