Add more tooltips in keybindings menu

**Is your feature request related to a problem? Please describe.**
It's not obvious what actions will actually do from just their label in the keybindings menu.

**Describe the solution you'd like**
Our keybindings menu would benefit from more/better tooltips. For example:
* What's the difference between 'Edit file' and 'Open file'?
* Does 'Execute custom command' refer to entering a command in from a prompt, or invoking a pre-defined custom command?
* Is there a way to change the scroll height of the 'Scroll up main panel' action?

Tooltips can answer all these questions.

If a given item is so self-explanatory that a tooltip is redundant, then we don't need to give it a tooltip.

**Describe alternatives you've considered**
None

**Additional context**
The existing keybinding labels are used in the cheatsheets. We should include tooltips in the cheatsheet as well.