UI: Defer loading hotkeys until tab is switched

[derrod]

Description

Waits with loading hotkeys until the user clicks on the settings tab.

The user sees a message while the tab is still loading:

Motivation and Context

Enables the settings window to still open really quickly even with tons of hotkeys (like #6081).

How Has This Been Tested?

Opened settings, it was quick!

Types of changes

• Tweak (non-breaking change to improve existing functionality)

Checklist:

• [x] My code has been run through clang-format.
• [x] I have read the contributing document.
• [x] My code is not on the master branch.
• [x] The code has been tested.
• [x] All commit messages are properly formatted and commits squashed where appropriate.
• [x] I have included updates to all appropriate documentation.