GSOC2020: Ctrl-shift-U does not work work when editor is focused #9895

[yzykov]

GSOC2020: Ctrl-shift-U does not work work when editor is focused #9895

1. I have tested the changes on Windows, Linux & Mac
2. I was following the existing style of the code. I noticed that SHORTCUT_SHIFT_KEY_MASK was missing and replaced with identical logic on line 1353. Therefore, I decided to create the variable.
3. According to [1] iBus is not supporting CTRL+SHIFT+U. Modified to use CTRL+ALT+U for Linux.
   a) No users were affected on Windows and Mac OS, because preexisting behaviour is the same for Windows and Mac OS.
   b) Users with Linux XIM/None settings are affected, because the hotkeys were changed.
4. Tested with XIM/None CTRL+SHIFT+U was working fine.
5. Tested with iBus/XIM/None CTRL+ALT+U is working on Linux.

[1] https://superuser.com/questions/358749/how-to-disable-ctrlshiftu-in-ubuntu-linux