Rename "welcome" to "home"

The topmost mode is called “welcome”, which gives a false impression of being useful only during onboarding / first time use scenarios.

Icon already corresponds to “home”, so only the name needs to be changed. No need to change the name internally, only the user-visible instances. 

Acceptance criteria: 

Qt Creator no longer utilizes the name "welcome" anywhere visible in the tool, documentation or actively maintained web pages.