Add super key support for thumb shift mode

[tychiu]

Symptom
Given that:

• typing method is set to thumb shift mode
• input mode is not set to any Latin mode
• a text field is in focus
• Super + Space remains the shortcut for switching to another input method or keyboard layout in IBus

When a user presses Super + Space
Then the user sees a space added to the text field and layout not switched

Expected
Given that:

• typing method is set to thumb shift mode
• input mode is not set to any Latin mode
• a text field is in focus
• Super + Space remains the shortcut for switching to another input method or keyboard layout in IBus

When a user presses Super + Space
Then the user sees that Anthy is no longer the active IBus Engine

Root Cause
The super key modifier is ignored only in thumb shift mode.

In Thumb Shift Mode (__process_key_event_thumb()):

state = state & (IBus.ModifierType.SHIFT_MASK |
                     IBus.ModifierType.CONTROL_MASK |
                     IBus.ModifierType.MOD1_MASK |
                     IBus.ModifierType.RELEASE_MASK)

In Romaji or Kana Mode (__process_key_event_internal2()):

state = state & (IBus.ModifierType.SHIFT_MASK |
                     IBus.ModifierType.CONTROL_MASK |
                     IBus.ModifierType.MOD1_MASK |
                     IBus.ModifierType.MOD4_MASK)

Solution
Supplement MOD4_MASK to the state usages specific to thumb shift mode.

Remarks
Please let me know if there are practices that I should follow.