imgui Keybinding screen unintuitive mouse support

[Brambor]

The mouse support is unintuitive:

1. Open the ? menu
2. Click Add keybinding
3. Click the blue highlight
4. Nothing happens
5. Click the text
6. It is correctly selected (popup add keybinding opens)

Expected:
When I click the highlight line in step 4, it pops up.
Otherwise, it seems unintuitive and broken.

Additional context:
In curses (legacy) UI, clicking anywhere on highlighted line does select the entry as expected.

• OS: Windows
  • OS Version: 10.0.19045.4046 (22H2)
• Game Version: 0.G-7376-gff3abac8e2-dirty [64-bit] --- I am actually at ff3abac, but not compiling tests makes it dirty and a different commit message, probably
• Graphics Version: Tiles
• Game Language: English [en]
• Mods loaded: [
  Dark Days Ahead [dda],
  Disable NPC Needs [no_npc_food],
  Portal Storms Ignore NPCs [personal_portal_storms],
  Slowdown Fungal Growth [no_fungal_growth]
  ]

Originally posted by @Brambor in #65709 (review)