Revert lazy initialization of the Text Editor

This essentially reverts 8c22f2ff but without
reverting most of the actual code changes since in the meantime other changes
depend on the adjusted API. Instead the initialization call is just done
synchronously from the constructor instead of through the QMetaObject API.

This fixes the problem of context menus not being shown in the editor most
of the time after opening a file that happened since the above mentioned
commit. Neither keyboard nor mouse clicks where triggering a context menu,
only after switching to another tab and back did things work correctly. Its
unclear to me why that happens, but I'd rather have a bit slower area
switching than non-working context menus.

CCBUG: 291052