restore popup messages; correct abusive "static" keyword for global variables

Some message boxes do not pop up becase GUI::Global::mainWindow has the unexpected value 0. This is because static, as used in GUI/Support/Tool/Globals.h, is not the right keyword to make a variable global [1].

Todo:

• repair GUI/Support/Tool/Globals.h
• check other static declarations

[1] https://stackoverflow.com/questions/14349877/static-global-variables-in-c

assigned to @j.wuttke

mentioned in merge request !1923 (merged)

closed with merge request !1923 (merged)