ux: add hotkey for OPEN EXTERNAL MERGETOOL button #3389

Job	Run time
Prepare version string	6s
Build / Build Linux (arm64)	3m 8s
Build / Build macOS (Apple Silicon)	3m 6s
Build / Build Linux	2m 59s
Build / Build Windows ARM64	3m 51s
Build / Build macOS (Intel)	4m 17s
Build / Build Windows x64	3m 38s
Package / Package Windows (win-x64)	18s
Package / Package Windows (win-arm64)	17s
Package / Package macOS (osx-x64)	20s
Package / Package macOS (osx-arm64)	20s
Package / Package Linux (linux-x64)	56s
Package / Package Linux (linux-arm64)	50s
	24m 6s

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