Factor out duplicated code and move to oomhelp
- src/oomhelp.cpp, src/oomhelp.h (get_oom_context_window): New.
- src/ribbon-callbacks.cpp (encryptInspector) (decryptAttachments, decryptInspector, startCertManager): Use get_oom_context_window.
- src/oomhelp.h, src/ribbon-callbacks.cpp (RELDISP): Move macro from ribbon-callbacks.cpp to oomhelp.h