DiffEditor: Some buttons are only available if QC's MainWindow is wide enough

Steps to reproduce:

• start QC with default size (or make it as small as possible)
• open a project that is a local repository or a checked out project
• change sth inside the project
• trigger a diff of this file / project

Current result:

• some buttons of the DiffEditor are only visible / usable if the MainWindow is wide enough (e.g. switching between Unified or Side-by-Side diff)

Expected result:

• buttons that aren't available should be usable by the '>>' menu included within the tool bar or similar