Cleanup old color names by replacing them with new ones.

[linuspahl]

Description

There were a few structure and name changes in sawmill the past, which lead to redundant colors.
Now we can cleanup Usage in the UI and remove some old color cariables in sawmill.

old -> new

global.textDefault -> text.primary
global.textSecondary -> text.secondary

global.background. -> background.primary
global.contentBackground -> background.content