Make sure single Alt key press is first handled by Qt

[Dmitry A. Kuminov]

The Alt key is intercepted by PM and used to activate the native PM window menu bar or the system menu if there is no native menu bar.

This steals the Alt key press from Qt and therefore prevents it from activating the Qt menu bar in Qt applications; the system menu is always activated instead. This doesn't make sense and needs to be fixed. For the system menu, there is a de-facto standard Alt+Space key combination which should also do the same in Qt applications but single Alt press should be normally passed to Qt which will cause the Qt menu bar activation if there is one.

[Dmitry A. Kuminov]

Done in r353.

Note that this also disables almost all system default shortcuts for frame windows such as Alt+F7 (Move), Alt+F10 (Maximize) and so on. This was done to let Qt applications process these key combinations normally. Note that Alt+F4, Alt+Esc and Ctrl+Esc will keep their functionality because of their obvious importance.

Note also that although Qt intercepts Alt+Space as well, it will activate the native system menu if the top-level window has one (i.e. has a title bar with the icon in the left corner) -- this is a handy shortcut which will also compensate for the lack of the direct ones like Alt+F7 (so that the user will still be able to move/resize/maximize the window w/o using the mouse).