Improve CodeMirror navigation

[andocz]

There are three closely related issues with navigation.

• [ ] CodeMirror's Ctrl+F shortcut only works when a node is selected in the active document. This leads to a frustrating scenario when switching to a new file using the file manager, as pressing Ctrl+F before selecting a node will activate the browser's search instead. It should be a global shortcut like it is in CodeChecker's web ui.
• [ ] The persistent search's shortcut (Alt+F) doesn't work at all for some reason. It only works when brought up via the console. Maybe we could bind it to Ctrl+F, like how it seems to be in CodeChecker's web UI. This would essentially fix #216.
• [ ] The goto addon seen on https://codemirror.net/demo/search.html would be great to have.