Client: Add a Command Palette to perform various actions

[jacobwod]

Imagine having a command palette in Hajk that would allow power users to quickly navigate the app.

I got the idea from VSCode's command palette and I think we should implement something very similar.

• It should activate on Cmd + K, as it seems to have become some kind of standard for opening a command palette (go ahead and google it)
• It should show a list of possible actions and filter the list as user starts typing
• If other shortcuts are available for the most common actions, those should be printed (just as in the screenshot below)

I got this idea when i noticed that I often must reach out for the trackpad only to focus the search field. So focusing the search field would be one obvious use case.

Another could be controlling plugins. Imagine this flow:

1. Cmd + K
2. Type "op" which filters the command list and focuses on the command called "Open tool…"
3. Press Enter
4. Now a list of available Hajk tools shows up in the command palette and user can either navigate with up/down arrows or filter by typing, e.g. "lay"
5. This filters down and focuses on the one tool that matches the input: LayerSwitcher
6. Press Enter which completes the command palette action, closes it and opens LayerSwitcher

OK, this seems like a lot in text but try it in a app that has a command palette. Takes 0.5 seconds at most. I really think this could be useful for power users.

This is slightly related to, but possibly even superseeds #257.