Proof tree view: Multiple small changes for readability #3012

Tests

• ✅ key.core.testgen:test [no report files] (13 succeeded)
• ✅ key.core:test [no report files] (2919 succeeded, 104 skipped)
• ✅ key.core:testProveRules [no report files] (196 succeeded)
• ✅ key.core:testRunAllFunProofs [no report files] (47 succeeded)
• ✅ key.core:testRunAllInfProofs [no report files] (30 succeeded)
• ✅ key.removegenerics:test [no report files] (45 succeeded)
• ✅ key.ui:test [no report files] (5 succeeded)
• ✅ key.util:test [no report files] (158 succeeded)
• ✅ keyext.exploration:test [no report files] (3 succeeded)
• ✅ recoder:test [no report files] (14 succeeded, 15 skipped)