add support to dynamically adjust file tree width

[TLINDEN]

Summary

• Closes [feature-request] add key bindings to adjust file tree size dynamically #147
• I have read the CONTIBUTING.md and AI_POLICY.md guides

How Did You Test this Change?

Manually.

ToDos

• Currently the resizing of the tree works, but the tree content is not updated, the tree pane is empty after the first resize operation. I am not sure, what causes this yet. Any help would be much appreciated.
• Also the right panel width needs to be updated, which is not implemented yet.
• the width increase/decrease step (currently hardcoded to 5) could be made configurable
• A shortcut could be added as well:
  • key w (for "wide") could set the width to a (configurable?) pre-set wider width as the default
  • key n (for "narrow") could do the reverse and set the width back to the configured default

[dlvhdr]

this doesn't seem to work. I checked it out and after resizing all the files disappear. Also the diff doesn't update it's size to re-wrap lines.