GitLab

- clear up some Q_OS_LINUX vs. Q_OS_UNIX
- clear up Q_WS_* vs. Q_OS_* (relatively theoretical exercise)
- use proper #if-#else cascades instead of free-standing #ifs

Reviewed-by: dt

Task:     - none
RevBy:    - Thorbjoern
AutoTest: - compile
Details:  - remove font settings stuff, since we overwrite it in coreplugin

Task:     - none
RevBy:    - TrustMe

Task:     - none
RevBy:    - me
AutoTest: - manual

Task:     - none
AutoTest: - compile test
Details:  - use QT_NO_WEBKIT instead of QT_WEBKIT, USE_WEBKIT